support generator functions (#4620)

12 files changed, 1178 insertions, 174 deletions

diff --git a/README.md b/README.md
index 9f69ce00..6884d8c8 100644
--- a/README.md
+++ b/README.md
@@ -811,6 +811,8 @@ to be `false` and all symbol names will be omitted.
 - `varify` (default: `true`) -- convert block-scoped declaractions into `var`
   whenever safe to do so
 
+- `yields` (default: `true`) -- apply optimizations to `yield` expressions
+
 ## Mangle options
 
 - `eval` (default `false`) -- Pass `true` to mangle names visible in scopes
diff --git a/lib/ast.js b/lib/ast.js
index ead2c7d5..ea823b28 100644
--- a/lib/ast.js
+++ b/lib/ast.js
@@ -113,7 +113,9 @@ var AST_Node = DEFNODE("Node", "start end", {
     walk: function(visitor) {
         visitor.visit(this);
     },
-    _validate: noop,
+    _validate: function() {
+        if (this.TYPE == "Node") throw new Error("should not instantiate AST_Node");
+    },
     validate: function() {
         var ctor = this.CTOR;
         do {
@@ -187,6 +189,9 @@ AST_Node.disable_validation = function() {
 
 var AST_Statement = DEFNODE("Statement", null, {
     $documentation: "Base class of all statements",
+    _validate: function() {
+        if (this.TYPE == "Statement") throw new Error("should not instantiate AST_Statement");
+    },
 });
 
 var AST_Debugger = DEFNODE("Debugger", null, {
@@ -215,7 +220,7 @@ function validate_expression(value, prop, multiple, allow_spread, allow_hole) {
     if (value instanceof AST_Destructured) throw new Error(prop + " cannot " + multiple + " AST_Destructured");
     if (value instanceof AST_Hole && !allow_hole) throw new Error(prop + " cannot " + multiple + " AST_Hole");
     if (value instanceof AST_Spread && !allow_spread) throw new Error(prop + " cannot " + multiple + " AST_Spread");
-    if (value instanceof AST_Statement && !is_function(value)) {
+    if (value instanceof AST_Statement && !(value instanceof AST_LambdaExpression)) {
         throw new Error(prop + " cannot " + multiple + " AST_Statement");
     }
     if (value instanceof AST_SymbolDeclaration) {
@@ -265,6 +270,7 @@ var AST_BlockScope = DEFNODE("BlockScope", "enclosed functions make_def parent_s
         return this.parent_scope.resolve();
     },
     _validate: function() {
+        if (this.TYPE == "BlockScope") throw new Error("should not instantiate AST_BlockScope");
         if (this.parent_scope == null) return;
         if (!(this.parent_scope instanceof AST_BlockScope)) throw new Error("parent_scope must be AST_BlockScope");
         if (!(this.resolve() instanceof AST_Scope)) throw new Error("must be contained within AST_Scope");
@@ -289,9 +295,10 @@ var AST_Block = DEFNODE("Block", "body", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "Block") throw new Error("should not instantiate AST_Block");
         this.body.forEach(function(node) {
             if (!(node instanceof AST_Statement)) throw new Error("body must be AST_Statement[]");
-            if (is_function(node)) throw new Error("body cannot contain AST_Function");
+            if (node instanceof AST_LambdaExpression) throw new Error("body cannot contain AST_LambdaExpression");
         });
     },
 }, AST_BlockScope);
@@ -306,8 +313,9 @@ var AST_StatementWithBody = DEFNODE("StatementWithBody", "body", {
         body: "[AST_Statement] the body; this should always be present, even if it's an AST_EmptyStatement"
     },
     _validate: function() {
+        if (this.TYPE == "StatementWithBody") throw new Error("should not instantiate AST_StatementWithBody");
         if (!(this.body instanceof AST_Statement)) throw new Error("body must be AST_Statement");
-        if (is_function(this.body)) throw new Error("body cannot be AST_Function");
+        if (this.body instanceof AST_LambdaExpression) throw new Error("body cannot be AST_LambdaExpression");
     },
 }, AST_BlockScope);
 
@@ -346,7 +354,10 @@ var AST_LabeledStatement = DEFNODE("LabeledStatement", "label", {
 }, AST_StatementWithBody);
 
 var AST_IterationStatement = DEFNODE("IterationStatement", null, {
-    $documentation: "Internal class.  All loops inherit from it."
+    $documentation: "Internal class.  All loops inherit from it.",
+    _validate: function() {
+        if (this.TYPE == "IterationStatement") throw new Error("should not instantiate AST_IterationStatement");
+    },
 }, AST_StatementWithBody);
 
 var AST_DWLoop = DEFNODE("DWLoop", "condition", {
@@ -355,6 +366,7 @@ var AST_DWLoop = DEFNODE("DWLoop", "condition", {
         condition: "[AST_Node] the loop condition.  Should not be instanceof AST_Statement"
     },
     _validate: function() {
+        if (this.TYPE == "DWLoop") throw new Error("should not instantiate AST_DWLoop");
         must_be_expression(this, "condition");
     },
 }, AST_IterationStatement);
@@ -401,7 +413,7 @@ var AST_For = DEFNODE("For", "init condition step", {
         if (this.init != null) {
             if (!(this.init instanceof AST_Node)) throw new Error("init must be AST_Node");
             if (this.init instanceof AST_Statement
-                && !(this.init instanceof AST_Definitions || is_function(this.init))) {
+                && !(this.init instanceof AST_Definitions || this.init instanceof AST_LambdaExpression)) {
                 throw new Error("init cannot be AST_Statement");
             }
         }
@@ -467,6 +479,9 @@ var AST_Scope = DEFNODE("Scope", "uses_eval uses_with", {
         return this.uses_eval || this.uses_with;
     },
     resolve: return_this,
+    _validate: function() {
+        if (this.TYPE == "Scope") throw new Error("should not instantiate AST_Scope");
+    },
 }, AST_Block);
 
 var AST_Toplevel = DEFNODE("Toplevel", "globals", {
@@ -517,8 +532,9 @@ var AST_Lambda = DEFNODE("Lambda", "argnames length_read rest uses_arguments", {
     $documentation: "Base class for functions",
     $propdoc: {
         argnames: "[(AST_DefaultValue|AST_Destructured|AST_SymbolFunarg)*] array of function arguments and/or destructured literals",
+        length_read: "[boolean/S] whether length property of this function is accessed",
         rest: "[(AST_Destructured|AST_SymbolFunarg)?] rest parameter, or null if absent",
-        uses_arguments: "[boolean/S] tells whether this function accesses the arguments array",
+        uses_arguments: "[boolean/S] whether this function accesses the arguments array",
     },
     each_argname: function(visit) {
         var tw = new TreeWalker(function(node) {
@@ -549,6 +565,7 @@ var AST_Lambda = DEFNODE("Lambda", "argnames length_read rest uses_arguments", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "Lambda") throw new Error("should not instantiate AST_Lambda");
         this.argnames.forEach(function(node) {
             validate_destructured(node, function(node) {
                 if (!(node instanceof AST_SymbolFunarg)) throw new Error("argnames must be AST_SymbolFunarg[]");
@@ -567,12 +584,33 @@ var AST_Accessor = DEFNODE("Accessor", null, {
     },
 }, AST_Lambda);
 
+var AST_LambdaExpression = DEFNODE("LambdaExpression", "inlined", {
+    $documentation: "Base class for function expressions",
+    $propdoc: {
+        inlined: "[boolean/S] whether this function has been inlined",
+    },
+    _validate: function() {
+        if (this.TYPE == "LambdaExpression") throw new Error("should not instantiate AST_LambdaExpression");
+    },
+}, AST_Lambda);
+
 function is_arrow(node) {
-    return node instanceof AST_AsyncArrow || node instanceof AST_Arrow;
+    return node instanceof AST_Arrow || node instanceof AST_AsyncArrow;
+}
+
+function is_async(node) {
+    return node instanceof AST_AsyncArrow
+        || node instanceof AST_AsyncDefun
+        || node instanceof AST_AsyncFunction
+        || node instanceof AST_AsyncGeneratorDefun
+        || node instanceof AST_AsyncGeneratorFunction;
 }
 
-function is_function(node) {
-    return is_arrow(node) || node instanceof AST_AsyncFunction || node instanceof AST_Function;
+function is_generator(node) {
+    return node instanceof AST_AsyncGeneratorDefun
+        || node instanceof AST_AsyncGeneratorFunction
+        || node instanceof AST_GeneratorDefun
+        || node instanceof AST_GeneratorFunction;
 }
 
 function walk_lambda(node, tw) {
@@ -583,7 +621,7 @@ function walk_lambda(node, tw) {
     }
 }
 
-var AST_Arrow = DEFNODE("Arrow", "inlined value", {
+var AST_Arrow = DEFNODE("Arrow", "value", {
     $documentation: "An arrow function expression",
     $propdoc: {
         value: "[AST_Node?] simple return expression, or null if using function body.",
@@ -610,13 +648,9 @@ var AST_Arrow = DEFNODE("Arrow", "inlined value", {
             if (this.body.length) throw new Error("body must be empty if value exists");
         }
     },
-}, AST_Lambda);
+}, AST_LambdaExpression);
 
-function is_async(node) {
-    return node instanceof AST_AsyncArrow || node instanceof AST_AsyncDefun || node instanceof AST_AsyncFunction;
-}
-
-var AST_AsyncArrow = DEFNODE("AsyncArrow", "inlined value", {
+var AST_AsyncArrow = DEFNODE("AsyncArrow", "value", {
     $documentation: "An asynchronous arrow function expression",
     $propdoc: {
         value: "[AST_Node?] simple return expression, or null if using function body.",
@@ -643,9 +677,9 @@ var AST_AsyncArrow = DEFNODE("AsyncArrow", "inlined value", {
             if (this.body.length) throw new Error("body must be empty if value exists");
         }
     },
-}, AST_Lambda);
+}, AST_LambdaExpression);
 
-var AST_AsyncFunction = DEFNODE("AsyncFunction", "inlined name", {
+var AST_AsyncFunction = DEFNODE("AsyncFunction", "name", {
     $documentation: "An asynchronous function expression",
     $propdoc: {
         name: "[AST_SymbolLambda?] the name of this function",
@@ -655,10 +689,10 @@ var AST_AsyncFunction = DEFNODE("AsyncFunction", "inlined name", {
             if (!(this.name instanceof AST_SymbolLambda)) throw new Error("name must be AST_SymbolLambda");
         }
     },
-}, AST_Lambda);
+}, AST_LambdaExpression);
 
-var AST_Function = DEFNODE("Function", "inlined name", {
-    $documentation: "A function expression",
+var AST_AsyncGeneratorFunction = DEFNODE("AsyncGeneratorFunction", "name", {
+    $documentation: "An asynchronous generator function expression",
     $propdoc: {
         name: "[AST_SymbolLambda?] the name of this function",
     },
@@ -667,36 +701,67 @@ var AST_Function = DEFNODE("Function", "inlined name", {
             if (!(this.name instanceof AST_SymbolLambda)) throw new Error("name must be AST_SymbolLambda");
         }
     },
-}, AST_Lambda);
+}, AST_LambdaExpression);
 
-function is_defun(node) {
-    return node instanceof AST_AsyncDefun || node instanceof AST_Defun;
-}
+var AST_Function = DEFNODE("Function", "name", {
+    $documentation: "A function expression",
+    $propdoc: {
+        name: "[AST_SymbolLambda?] the name of this function",
+    },
+    _validate: function() {
+        if (this.name != null) {
+            if (!(this.name instanceof AST_SymbolLambda)) throw new Error("name must be AST_SymbolLambda");
+        }
+    },
+}, AST_LambdaExpression);
 
-var AST_AsyncDefun = DEFNODE("AsyncDefun", "inlined name", {
-    $documentation: "An asynchronous function definition",
+var AST_GeneratorFunction = DEFNODE("GeneratorFunction", "name", {
+    $documentation: "A generator function expression",
     $propdoc: {
-        name: "[AST_SymbolDefun] the name of this function",
+        name: "[AST_SymbolLambda?] the name of this function",
     },
     _validate: function() {
-        if (!(this.name instanceof AST_SymbolDefun)) throw new Error("name must be AST_SymbolDefun");
+        if (this.name != null) {
+            if (!(this.name instanceof AST_SymbolLambda)) throw new Error("name must be AST_SymbolLambda");
+        }
     },
-}, AST_Lambda);
+}, AST_LambdaExpression);
 
-var AST_Defun = DEFNODE("Defun", "inlined name", {
-    $documentation: "A function definition",
+var AST_LambdaDefinition = DEFNODE("LambdaDefinition", "inlined name", {
+    $documentation: "Base class for function definitions",
     $propdoc: {
+        inlined: "[boolean/S] whether this function has been inlined",
         name: "[AST_SymbolDefun] the name of this function",
     },
     _validate: function() {
+        if (this.TYPE == "LambdaDefinition") throw new Error("should not instantiate AST_LambdaDefinition");
         if (!(this.name instanceof AST_SymbolDefun)) throw new Error("name must be AST_SymbolDefun");
     },
 }, AST_Lambda);
 
+var AST_AsyncDefun = DEFNODE("AsyncDefun", null, {
+    $documentation: "An asynchronous function definition",
+}, AST_LambdaDefinition);
+
+var AST_AsyncGeneratorDefun = DEFNODE("AsyncGeneratorDefun", null, {
+    $documentation: "An asynchronous generator function definition",
+}, AST_LambdaDefinition);
+
+var AST_Defun = DEFNODE("Defun", null, {
+    $documentation: "A function definition",
+}, AST_LambdaDefinition);
+
+var AST_GeneratorDefun = DEFNODE("GeneratorDefun", null, {
+    $documentation: "A generator function definition",
+}, AST_LambdaDefinition);
+
 /* -----[ JUMPS ]----- */
 
 var AST_Jump = DEFNODE("Jump", null, {
-    $documentation: "Base class for “jumps” (for now that's `return`, `throw`, `break` and `continue`)"
+    $documentation: "Base class for “jumps” (for now that's `return`, `throw`, `break` and `continue`)",
+    _validate: function() {
+        if (this.TYPE == "Jump") throw new Error("should not instantiate AST_Jump");
+    },
 }, AST_Statement);
 
 var AST_Exit = DEFNODE("Exit", "value", {
@@ -709,7 +774,10 @@ var AST_Exit = DEFNODE("Exit", "value", {
         visitor.visit(node, function() {
             if (node.value) node.value.walk(visitor);
         });
-    }
+    },
+    _validate: function() {
+        if (this.TYPE == "Exit") throw new Error("should not instantiate AST_Exit");
+    },
 }, AST_Jump);
 
 var AST_Return = DEFNODE("Return", null, {
@@ -738,6 +806,7 @@ var AST_LoopControl = DEFNODE("LoopControl", "label", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "LoopControl") throw new Error("should not instantiate AST_LoopControl");
         if (this.label != null) {
             if (!(this.label instanceof AST_LabelRef)) throw new Error("label must be AST_LabelRef");
         }
@@ -772,7 +841,7 @@ var AST_If = DEFNODE("If", "condition alternative", {
         must_be_expression(this, "condition");
         if (this.alternative != null) {
             if (!(this.alternative instanceof AST_Statement)) throw new Error("alternative must be AST_Statement");
-            if (is_function(this.alternative)) throw new error("alternative cannot be AST_Function");
+            if (this.alternative instanceof AST_LambdaExpression) throw new error("alternative cannot be AST_LambdaExpression");
         }
     },
 }, AST_StatementWithBody);
@@ -801,6 +870,9 @@ var AST_Switch = DEFNODE("Switch", "expression", {
 
 var AST_SwitchBranch = DEFNODE("SwitchBranch", null, {
     $documentation: "Base class for `switch` branches",
+    _validate: function() {
+        if (this.TYPE == "SwitchBranch") throw new Error("should not instantiate AST_SwitchBranch");
+    },
 }, AST_Block);
 
 var AST_Default = DEFNODE("Default", null, {
@@ -889,6 +961,7 @@ var AST_Definitions = DEFNODE("Definitions", "definitions", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "Definitions") throw new Error("should not instantiate AST_Definitions");
         if (this.definitions.length < 1) throw new Error("must have at least one definition");
     },
 }, AST_Statement);
@@ -1036,6 +1109,7 @@ var AST_PropAccess = DEFNODE("PropAccess", "expression property", {
         return p;
     },
     _validate: function() {
+        if (this.TYPE == "PropAccess") throw new Error("should not instantiate AST_PropAccess");
         must_be_expression(this, "expression");
     },
 });
@@ -1096,6 +1170,7 @@ var AST_Unary = DEFNODE("Unary", "operator expression", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "Unary") throw new Error("should not instantiate AST_Unary");
         if (typeof this.operator != "string") throw new Error("operator must be string");
         must_be_expression(this, "expression");
     },
@@ -1183,6 +1258,27 @@ var AST_Await = DEFNODE("Await", "expression", {
     },
 });
 
+var AST_Yield = DEFNODE("Yield", "expression nested", {
+    $documentation: "A yield expression",
+    $propdoc: {
+        expression: "[AST_Node?] return value for iterator, or null if undefined",
+        nested: "[boolean] whether to iterate over expression as generator",
+    },
+    walk: function(visitor) {
+        var node = this;
+        visitor.visit(node, function() {
+            if (node.expression) node.expression.walk(visitor);
+        });
+    },
+    _validate: function() {
+        if (this.expression != null) {
+            must_be_expression(this, "expression");
+        } else if (this.nested) {
+            throw new Error("yield* must contain expression");
+        }
+    },
+});
+
 /* -----[ LITERALS ]----- */
 
 var AST_Array = DEFNODE("Array", "elements", {
@@ -1208,6 +1304,9 @@ var AST_Destructured = DEFNODE("Destructured", "rest", {
     $propdoc: {
         rest: "[(AST_Destructured|AST_SymbolDeclaration|AST_SymbolRef)?] rest parameter, or null if absent",
     },
+    _validate: function() {
+        if (this.TYPE == "Destructured") throw new Error("should not instantiate AST_Destructured");
+    },
 });
 
 function validate_destructured(node, check, allow_default) {
@@ -1319,6 +1418,7 @@ var AST_ObjectProperty = DEFNODE("ObjectProperty", "key value", {
         });
     },
     _validate: function() {
+        if (this.TYPE == "ObjectProperty") throw new Error("should not instantiate AST_ObjectProperty");
         if (typeof this.key != "string") {
             if (!(this.key instanceof AST_Node)) throw new Error("key must be string or AST_Node");
             must_be_expression(this, "key");
@@ -1356,6 +1456,7 @@ var AST_Symbol = DEFNODE("Symbol", "scope name thedef", {
         thedef: "[SymbolDef/S] the definition of this symbol"
     },
     _validate: function() {
+        if (this.TYPE == "Symbol") throw new Error("should not instantiate AST_Symbol");
         if (typeof this.name != "string") throw new Error("name must be string");
     },
 });
@@ -1448,6 +1549,9 @@ var AST_Template = DEFNODE("Template", "expressions strings tag", {
 
 var AST_Constant = DEFNODE("Constant", null, {
     $documentation: "Base class for all constants",
+    _validate: function() {
+        if (this.TYPE == "Constant") throw new Error("should not instantiate AST_Constant");
+    },
 });
 
 var AST_String = DEFNODE("String", "value quote", {
@@ -1496,6 +1600,9 @@ var AST_RegExp = DEFNODE("RegExp", "value", {
 
 var AST_Atom = DEFNODE("Atom", null, {
     $documentation: "Base class for atoms",
+    _validate: function() {
+        if (this.TYPE == "Atom") throw new Error("should not instantiate AST_Atom");
+    },
 }, AST_Constant);
 
 var AST_Null = DEFNODE("Null", null, {
@@ -1525,6 +1632,9 @@ var AST_Infinity = DEFNODE("Infinity", null, {
 
 var AST_Boolean = DEFNODE("Boolean", null, {
     $documentation: "Base class for booleans",
+    _validate: function() {
+        if (this.TYPE == "Boolean") throw new Error("should not instantiate AST_Boolean");
+    },
 }, AST_Atom);
 
 var AST_False = DEFNODE("False", null, {
diff --git a/lib/compress.js b/lib/compress.js
index afb555f0..9e9f15f6 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -104,6 +104,7 @@ function Compressor(options, false_by_default) {
         unsafe_undefined: false,
         unused          : !false_by_default,
         varify          : !false_by_default,
+        yields          : !false_by_default,
     }, true);
     var evaluate = this.options["evaluate"];
     this.eval_threshold = /eager/.test(evaluate) ? 1 / 0 : +evaluate;
@@ -344,7 +345,7 @@ merge(Compressor.prototype, {
             return !immutable
                 && parent.expression === node
                 && !parent.is_expr_pure(compressor)
-                && (!is_function(value) || !(parent instanceof AST_New) && value.contains_this());
+                && (!(value instanceof AST_LambdaExpression) || !(parent instanceof AST_New) && value.contains_this());
         }
         if (parent instanceof AST_ForIn) return parent.init === node;
         if (parent instanceof AST_ObjectKeyVal) {
@@ -396,13 +397,13 @@ merge(Compressor.prototype, {
             def.fixed = !def.const_redefs
                 && !def.scope.pinned()
                 && !compressor.exposed(def)
-                && !(is_function(def.init) && def.init !== def.scope)
+                && !(def.init instanceof AST_LambdaExpression && def.init !== def.scope)
                 && def.init;
-            if (is_defun(def.fixed) && !all(def.references, function(ref) {
+            if (def.fixed instanceof AST_LambdaDefinition && !all(def.references, function(ref) {
                 var scope = ref.scope.resolve();
                 do {
                     if (def.scope === scope) return true;
-                } while (is_function(scope) && (scope = scope.parent_scope.resolve()));
+                } while (scope instanceof AST_LambdaExpression && (scope = scope.parent_scope.resolve()));
             })) {
                 tw.defun_ids[def.id] = false;
             }
@@ -427,7 +428,7 @@ merge(Compressor.prototype, {
                 scope.may_call_this = noop;
                 if (!scope.contains_this()) return;
                 scope.functions.each(function(def) {
-                    if (is_defun(def.init) && !(def.id in tw.defun_ids)) {
+                    if (def.init instanceof AST_LambdaDefinition && !(def.id in tw.defun_ids)) {
                         tw.defun_ids[def.id] = false;
                     }
                 });
@@ -467,7 +468,7 @@ merge(Compressor.prototype, {
 
         function walk_defuns(tw, scope) {
             scope.functions.each(function(def) {
-                if (is_defun(def.init) && !tw.defun_visited[def.id]) {
+                if (def.init instanceof AST_LambdaDefinition && !tw.defun_visited[def.id]) {
                     tw.defun_ids[def.id] = tw.safe_ids;
                     def.init.walk(tw);
                 }
@@ -505,7 +506,7 @@ merge(Compressor.prototype, {
                 }
                 return !safe.assign || safe.assign === tw.safe_ids;
             }
-            return is_defun(def.fixed);
+            return def.fixed instanceof AST_LambdaDefinition;
         }
 
         function safe_to_assign(tw, def, declare) {
@@ -702,25 +703,11 @@ merge(Compressor.prototype, {
             lhs.walk(scanner);
         }
 
-        function reduce_defun(tw, descend, compressor) {
-            var id = this.name.definition().id;
-            if (tw.defun_visited[id]) return true;
-            if (tw.defun_ids[id] !== tw.safe_ids) return true;
-            tw.defun_visited[id] = true;
-            this.inlined = false;
-            push(tw);
-            reset_variables(tw, compressor, this);
-            descend();
-            pop(tw);
-            walk_defuns(tw, this);
-            return true;
-        }
-
         function reduce_iife(tw, descend, compressor) {
             var fn = this;
             fn.inlined = false;
             var iife = tw.parent();
-            var hit = is_async(fn);
+            var hit = is_async(fn) || is_generator(fn);
             var aborts = false;
             fn.walk(new TreeWalker(function(node) {
                 if (hit) return aborts = true;
@@ -848,7 +835,6 @@ merge(Compressor.prototype, {
                 }
             }
         });
-        def(AST_AsyncDefun, reduce_defun);
         def(AST_Binary, function(tw) {
             if (!lazy_op[this.operator]) return;
             this.left.walk(tw);
@@ -865,7 +851,7 @@ merge(Compressor.prototype, {
         def(AST_Call, function(tw, descend) {
             tw.find_parent(AST_Scope).may_call_this();
             var exp = this.expression;
-            if (is_function(exp)) {
+            if (exp instanceof AST_LambdaExpression) {
                 var iife = !exp.name;
                 this.args.forEach(function(arg) {
                     arg.walk(tw);
@@ -878,7 +864,7 @@ merge(Compressor.prototype, {
             } else if (exp instanceof AST_SymbolRef) {
                 var def = exp.definition();
                 if (this.TYPE == "Call" && tw.in_boolean_context()) def.bool_fn++;
-                if (!is_defun(def.fixed)) return;
+                if (!(def.fixed instanceof AST_LambdaDefinition)) return;
                 var defun = mark_defun(tw, def);
                 if (!defun) return;
                 descend();
@@ -909,7 +895,6 @@ merge(Compressor.prototype, {
             pop(tw);
             return true;
         });
-        def(AST_Defun, reduce_defun);
         def(AST_Do, function(tw) {
             var saved_loop = tw.in_loop;
             tw.in_loop = this;
@@ -1008,6 +993,19 @@ merge(Compressor.prototype, {
             walk_defuns(tw, fn);
             return true;
         });
+        def(AST_LambdaDefinition, function(tw, descend, compressor) {
+            var id = this.name.definition().id;
+            if (tw.defun_visited[id]) return true;
+            if (tw.defun_ids[id] !== tw.safe_ids) return true;
+            tw.defun_visited[id] = true;
+            this.inlined = false;
+            push(tw);
+            reset_variables(tw, compressor, this);
+            descend();
+            pop(tw);
+            walk_defuns(tw, this);
+            return true;
+        });
         def(AST_Switch, function(tw, descend, compressor) {
             this.variables.each(function(def) {
                 reset_def(tw, compressor, def);
@@ -1079,7 +1077,8 @@ merge(Compressor.prototype, {
             }
             if (!this.fixed) this.fixed = d.fixed;
             var parent;
-            if (is_defun(d.fixed) && !((parent = tw.parent()) instanceof AST_Call && parent.expression === this)) {
+            if (d.fixed instanceof AST_LambdaDefinition
+                && !((parent = tw.parent()) instanceof AST_Call && parent.expression === this)) {
                 var defun = mark_defun(tw, d);
                 if (defun) defun.walk(tw);
             }
@@ -1455,8 +1454,10 @@ merge(Compressor.prototype, {
 
     function is_iife_call(node) {
         if (node.TYPE != "Call") return false;
-        var exp = node.expression;
-        return exp instanceof AST_AsyncFunction || exp instanceof AST_Function || is_iife_call(exp);
+        do {
+            node = node.expression;
+        } while (node instanceof AST_PropAccess);
+        return node instanceof AST_LambdaExpression ? !is_arrow(node) : is_iife_call(node);
     }
 
     function is_undeclared_ref(node) {
@@ -1843,7 +1844,7 @@ merge(Compressor.prototype, {
                 if (node instanceof AST_Call) {
                     if (!(lhs instanceof AST_PropAccess)) return false;
                     if (!lhs.equivalent_to(node.expression)) return false;
-                    return !(is_function(rvalue) && !rvalue.contains_this());
+                    return !(rvalue instanceof AST_LambdaExpression && !rvalue.contains_this());
                 }
                 if (node instanceof AST_Debugger) return true;
                 if (node instanceof AST_Defun) return funarg && lhs.name === node.name.name;
@@ -1951,6 +1952,7 @@ merge(Compressor.prototype, {
                             && (lvalues.has(node.name) || side_effects && may_modify(node));
                     }, true);
                 }
+                if (node instanceof AST_Yield) return true;
                 var sym = is_lhs(node.left, node);
                 if (!sym) return false;
                 if (sym instanceof AST_PropAccess) return true;
@@ -1988,7 +1990,8 @@ merge(Compressor.prototype, {
 
             function extract_args() {
                 var iife, fn = compressor.self();
-                if (is_function(fn)
+                if (fn instanceof AST_LambdaExpression
+                    && !is_generator(fn)
                     && !fn.name
                     && !fn.uses_arguments
                     && !fn.pinned()
@@ -2482,7 +2485,7 @@ merge(Compressor.prototype, {
                         if (modify_toplevel) return;
                         var exp = node.expression;
                         if (exp instanceof AST_PropAccess) return;
-                        if (is_function(exp) && !exp.contains_this()) return;
+                        if (exp instanceof AST_LambdaExpression && !exp.contains_this()) return;
                         modify_toplevel = true;
                     } else if (node instanceof AST_PropAccess && may_be_global(node.expression)) {
                         if (node === lhs && !(expr instanceof AST_Unary)) {
@@ -3249,7 +3252,7 @@ merge(Compressor.prototype, {
     }
 
     function extract_declarations_from_unreachable_code(compressor, stat, target) {
-        if (!(stat instanceof AST_Definitions || is_defun(stat))) {
+        if (!(stat instanceof AST_Definitions || stat instanceof AST_LambdaDefinition)) {
             AST_Node.warn("Dropping unreachable code [{file}:{line},{col}]", stat.start);
         }
         var block;
@@ -3265,7 +3268,7 @@ merge(Compressor.prototype, {
                 }
                 return true;
             }
-            if (is_defun(node)) {
+            if (node instanceof AST_LambdaDefinition) {
                 push(node);
                 return true;
             }
@@ -5114,7 +5117,7 @@ merge(Compressor.prototype, {
             if (node instanceof AST_Call) {
                 var exp = node.expression;
                 var tail = exp.tail_node();
-                if (!is_function(tail)) return;
+                if (!(tail instanceof AST_LambdaExpression)) return;
                 if (exp !== tail) exp.expressions.slice(0, -1).forEach(function(node) {
                     node.walk(tw);
                 });
@@ -5534,7 +5537,7 @@ merge(Compressor.prototype, {
             }
             if (node === self) return;
             if (scope === self) {
-                if (is_defun(node)) {
+                if (node instanceof AST_LambdaDefinition) {
                     var def = node.name.definition();
                     if (!drop_funcs && !(def.id in in_use_ids)) {
                         in_use_ids[def.id] = true;
@@ -5735,7 +5738,7 @@ merge(Compressor.prototype, {
             if (node instanceof AST_Call) calls_to_drop_args.push(node);
             if (scope !== self) return;
             if (node instanceof AST_Lambda) {
-                if (drop_funcs && node !== self && is_defun(node)) {
+                if (drop_funcs && node !== self && node instanceof AST_LambdaDefinition) {
                     var def = node.name.definition();
                     if (!(def.id in in_use_ids)) {
                         log(node.name, "Dropping unused function {name}");
@@ -5743,7 +5746,7 @@ merge(Compressor.prototype, {
                         return in_list ? List.skip : make_node(AST_EmptyStatement, node);
                     }
                 }
-                if (is_function(node) && node.name && drop_fn_name(node.name.definition())) {
+                if (node instanceof AST_LambdaExpression && node.name && drop_fn_name(node.name.definition())) {
                     unused_fn_names.push(node);
                 }
                 if (!(node instanceof AST_Accessor)) {
@@ -5853,12 +5856,28 @@ merge(Compressor.prototype, {
                             && node instanceof AST_Var
                             && var_defs[sym.id] == 1
                             && sym.assignments == 0
-                            && (value instanceof AST_AsyncFunction || value instanceof AST_Function)
+                            && value instanceof AST_LambdaExpression
+                            && !is_arrow(value)
                             && assigned_once(value, sym.references)
                             && can_declare_defun()
                             && can_rename(value, def.name.name)) {
                             AST_Node.warn("Declaring {name} as function [{file}:{line},{col}]", template(def.name));
-                            var defun = make_node(value instanceof AST_Function ? AST_Defun : AST_AsyncDefun, def, value);
+                            var ctor;
+                            switch (value.CTOR) {
+                              case AST_AsyncFunction:
+                                ctor = AST_AsyncDefun;
+                                break;
+                              case AST_AsyncGeneratorFunction:
+                                ctor = AST_AsyncGeneratorDefun;
+                                break;
+                              case AST_Function:
+                                ctor = AST_Defun;
+                                break;
+                              case AST_GeneratorFunction:
+                                ctor = AST_GeneratorDefun;
+                                break;
+                            }
+                            var defun = make_node(ctor, def, value);
                             defun.name = make_node(AST_SymbolDefun, def.name, def.name);
                             var name_def = def.name.scope.resolve().def_function(defun.name);
                             if (old_def) old_def.forEach(function(node) {
@@ -5916,6 +5935,7 @@ merge(Compressor.prototype, {
                         if (old_def.assignments > 0) return false;
                         if (old_def.name == name) return true;
                         if (name == "await" && is_async(fn)) return false;
+                        if (name == "yield" && is_generator(fn)) return false;
                         return all(old_def.references, function(ref) {
                             return ref.scope.find_variable(name) === sym;
                         });
@@ -6838,7 +6858,6 @@ merge(Compressor.prototype, {
             })) return this;
             return make_sequence(this, values.map(convert_spread));
         });
-        def(AST_Arrow, return_null);
         def(AST_Assign, function(compressor) {
             var left = this.left;
             if (left instanceof AST_PropAccess) {
@@ -6855,8 +6874,6 @@ merge(Compressor.prototype, {
             }
             return this;
         });
-        def(AST_AsyncArrow, return_null);
-        def(AST_AsyncFunction, return_null);
         def(AST_Await, function(compressor) {
             if (!compressor.option("awaits")) return this;
             var exp = this.expression.drop_side_effect_free(compressor);
@@ -6991,6 +7008,7 @@ merge(Compressor.prototype, {
         def(AST_Function, function(compressor) {
             return fn_name_unused(this, compressor) ? null : this;
         });
+        def(AST_LambdaExpression, return_null);
         def(AST_Object, function(compressor, first_in_statement) {
             var exprs = [];
             this.properties.forEach(function(prop) {
@@ -7553,7 +7571,7 @@ merge(Compressor.prototype, {
                 var exprs = [];
                 for (var i = 0; i < stat.body.length; i++) {
                     var line = stat.body[i];
-                    if (is_defun(line)) {
+                    if (line instanceof AST_LambdaDefinition) {
                         defuns.push(line);
                     } else if (line instanceof AST_EmptyStatement) {
                         continue;
@@ -7570,7 +7588,7 @@ merge(Compressor.prototype, {
                 }
                 return exprs;
             }
-            if (is_defun(stat)) {
+            if (stat instanceof AST_LambdaDefinition) {
                 defuns.push(stat);
                 return [];
             }
@@ -8274,8 +8292,10 @@ merge(Compressor.prototype, {
             }
         }
         var fn = exp instanceof AST_SymbolRef ? exp.fixed_value() : exp;
-        var is_func = fn instanceof AST_Lambda && (!is_async(fn)
-            || compressor.option("awaits") && compressor.parent() instanceof AST_Await);
+        var parent = compressor.parent(), current = compressor.self();
+        var is_func = fn instanceof AST_Lambda
+            && (!is_async(fn) || compressor.option("awaits") && parent instanceof AST_Await)
+            && (!is_generator(fn) || compressor.option("yields") && current instanceof AST_Yield && current.nested);
         var stat = is_func && fn.first_statement();
         var has_default = 0, has_destructured = false;
         var has_spread = !all(self.args, function(arg) {
@@ -8308,7 +8328,7 @@ merge(Compressor.prototype, {
             if (can_inline
                 && !fn.uses_arguments
                 && !fn.pinned()
-                && !(fn.name && is_function(fn))
+                && !(fn.name && fn instanceof AST_LambdaExpression)
                 && (exp === fn || !recursive_ref(compressor, def = exp.definition())
                     && fn.is_constant_expression(find_scope(compressor)))
                 && !has_spread
@@ -8339,7 +8359,7 @@ merge(Compressor.prototype, {
                         return arg;
                     })).optimize(compressor);
                     fn.inlined = save_inlined;
-                    node = maintain_this_binding(compressor, compressor.parent(), compressor.self(), node);
+                    node = maintain_this_binding(compressor, parent, current, node);
                     if (replacing || best_of_expression(node, self) === node) {
                         refs.forEach(function(ref) {
                             var def = ref.definition();
@@ -8358,7 +8378,7 @@ merge(Compressor.prototype, {
                     fn._squeezed = true;
                     if (exp !== fn) fn.parent_scope = exp.scope;
                     var node = make_sequence(self, flatten_fn()).optimize(compressor);
-                    return maintain_this_binding(compressor, compressor.parent(), compressor.self(), node);
+                    return maintain_this_binding(compressor, parent, current, node);
                 }
             }
             if (compressor.option("side_effects")
@@ -8380,9 +8400,7 @@ merge(Compressor.prototype, {
                 }
             }
         }
-        if (compressor.option("negate_iife")
-            && compressor.parent() instanceof AST_SimpleStatement
-            && is_iife_call(self)) {
+        if (compressor.option("negate_iife") && parent instanceof AST_SimpleStatement && is_iife_call(current)) {
             return self.negate(compressor, true);
         }
         return try_evaluate(compressor, self);
@@ -8480,7 +8498,7 @@ merge(Compressor.prototype, {
                     if (node === fn) return;
                     if (is_arrow(node)) {
                         for (var i = 0; safe && i < node.argnames.length; i++) node.argnames[i].walk(tw);
-                    } else if (is_defun(node) && node.name.name == "await") {
+                    } else if (node instanceof AST_LambdaDefinition && node.name.name == "await") {
                         safe = false;
                     }
                     return true;
@@ -8566,7 +8584,7 @@ merge(Compressor.prototype, {
                         in_order = null;
                         return;
                     }
-                    if (is_defun(def.init)) return abort = true;
+                    if (def.init instanceof AST_LambdaDefinition) return abort = true;
                     if (is_lhs(node, this.parent())) return abort = true;
                     var index = resolve_index(def);
                     if (!(begin < index)) begin = index;
@@ -8619,7 +8637,7 @@ merge(Compressor.prototype, {
         function can_inject_vars(defined, used, safe_to_inject) {
             for (var i = 0; i < fn.body.length; i++) {
                 var stat = fn.body[i];
-                if (is_defun(stat)) {
+                if (stat instanceof AST_LambdaDefinition) {
                     if (!safe_to_inject || var_exists(used, stat.name.name)) return false;
                     if (!all(stat.enclosed, function(def) {
                         return def.scope === stat || !defined[def.name];
@@ -8640,7 +8658,7 @@ merge(Compressor.prototype, {
         function can_inject_symbols() {
             var defined = Object.create(null);
             var level = 0, child;
-            scope = compressor.self();
+            scope = current;
             do {
                 if (scope.variables) scope.variables.each(function(def) {
                     defined[def.name] = true;
@@ -8806,7 +8824,7 @@ merge(Compressor.prototype, {
             flatten_vars(decls, expressions);
             expressions.push(value);
             var args = fn.body.filter(function(stat) {
-                if (is_defun(stat)) {
+                if (stat instanceof AST_LambdaDefinition) {
                     var def = stat.name.definition();
                     scope.functions.set(def.name, def);
                     scope.variables.set(def.name, def);
@@ -9040,6 +9058,20 @@ merge(Compressor.prototype, {
         return self;
     });
 
+    OPT(AST_Yield, function(self, compressor) {
+        if (!compressor.option("yields")) return self;
+        if (compressor.option("sequences")) {
+            var seq = lift_sequence_in_expression(self, compressor);
+            if (seq !== self) return seq.optimize(compressor);
+        }
+        var exp = self.expression;
+        if (self.nested && exp.TYPE == "Call") {
+            var inlined = exp.clone().optimize(compressor);
+            if (inlined.TYPE != "Call") return inlined;
+        }
+        return self;
+    });
+
     AST_Binary.DEFMETHOD("lift_sequences", function(compressor) {
         if (this.left instanceof AST_PropAccess) {
             if (!(this.left.expression instanceof AST_Sequence)) return this;
@@ -9802,7 +9834,9 @@ merge(Compressor.prototype, {
                         if (single_use == "f") {
                             var scope = self.scope;
                             do {
-                                if (is_defun(scope) || is_function(scope)) scope.inlined = true;
+                                if (scope instanceof AST_LambdaDefinition || scope instanceof AST_LambdaExpression) {
+                                    scope.inlined = true;
+                                }
                             } while (scope = scope.parent_scope);
                         }
                     } else if (fixed.name && fixed.name.name == "await" && is_async(fixed)) {
@@ -9817,11 +9851,23 @@ merge(Compressor.prototype, {
                 def.single_use = false;
                 fixed._squeezed = true;
                 fixed.single_use = true;
-                if (fixed instanceof AST_AsyncDefun) {
-                    fixed = make_node(AST_AsyncFunction, fixed, fixed);
-                    fixed.name = make_node(AST_SymbolLambda, fixed.name, fixed.name);
-                } else if (fixed instanceof AST_Defun) {
-                    fixed = make_node(AST_Function, fixed, fixed);
+                if (fixed instanceof AST_LambdaDefinition) {
+                    var ctor;
+                    switch (fixed.CTOR) {
+                      case AST_AsyncDefun:
+                        ctor = AST_AsyncFunction;
+                        break;
+                      case AST_AsyncGeneratorDefun:
+                        ctor = AST_AsyncGeneratorFunction;
+                        break;
+                      case AST_Defun:
+                        ctor = AST_Function;
+                        break;
+                      case AST_GeneratorDefun:
+                        ctor = AST_GeneratorFunction;
+                        break;
+                    }
+                    fixed = make_node(ctor, fixed, fixed);
                     fixed.name = make_node(AST_SymbolLambda, fixed.name, fixed.name);
                 }
                 if (fixed instanceof AST_Lambda) {
diff --git a/lib/output.js b/lib/output.js
index a171c7e7..b2213f74 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -673,7 +673,9 @@ function OutputStream(options) {
         }
     }
     PARENS(AST_AsyncFunction, needs_parens_function);
+    PARENS(AST_AsyncGeneratorFunction, needs_parens_function);
     PARENS(AST_Function, needs_parens_function);
+    PARENS(AST_GeneratorFunction, needs_parens_function);
 
     // same goes for an object literal, because otherwise it would be
     // interpreted as a block of code.
@@ -682,14 +684,19 @@ function OutputStream(options) {
     }
     PARENS(AST_Object, needs_parens_obj);
 
-    PARENS(AST_Unary, function(output) {
+    function needs_parens_unary(output) {
         var p = output.parent();
         // (-x) ** y
         if (p instanceof AST_Binary) return p.operator == "**" && p.left === this;
-        // (x++).toString(3)
-        // (typeof x).length
-        return (p instanceof AST_Call || p instanceof AST_PropAccess) && p.expression === this;
-    });
+        // (await x)(y)
+        // new (await x)
+        if (p instanceof AST_Call) return p.expression === this;
+        // (x++)[y]
+        // (typeof x).y
+        if (p instanceof AST_PropAccess) return p.expression === this;
+    }
+    PARENS(AST_Await, needs_parens_unary);
+    PARENS(AST_Unary, needs_parens_unary);
 
     PARENS(AST_Sequence, function(output) {
         var p = output.parent();
@@ -719,7 +726,9 @@ function OutputStream(options) {
             // !(foo, bar, baz)
             || p instanceof AST_Unary
             // var a = (1, 2), b = a + a; ---> b == 4
-            || p instanceof AST_VarDef;
+            || p instanceof AST_VarDef
+            // yield (foo, bar)
+            || p instanceof AST_Yield;
     });
 
     PARENS(AST_Binary, function(output) {
@@ -826,17 +835,8 @@ function OutputStream(options) {
     PARENS(AST_Conditional, function(output) {
         return needs_parens_assign_cond(this, output);
     });
-
-    PARENS(AST_Await, function(output) {
-        var p = output.parent();
-        // (await x) ** y
-        if (p instanceof AST_Binary) return p.operator == "**" && p.left === this;
-        // new (await foo)
-        // (await foo)(bar)
-        if (p instanceof AST_Call) return p.expression === this;
-        // (await foo).prop
-        // (await foo)["prop"]
-        if (p instanceof AST_PropAccess) return p.expression === this;
+    PARENS(AST_Yield, function(output) {
+        return needs_parens_assign_cond(this, output);
     });
 
     /* -----[ PRINTERS ]----- */
@@ -1061,6 +1061,20 @@ function OutputStream(options) {
     }
     DEFPRINT(AST_AsyncDefun, print_async);
     DEFPRINT(AST_AsyncFunction, print_async);
+    function print_async_generator(output) {
+        output.print("async");
+        output.space();
+        output.print("function*");
+        print_lambda(this, output);
+    }
+    DEFPRINT(AST_AsyncGeneratorDefun, print_async_generator);
+    DEFPRINT(AST_AsyncGeneratorFunction, print_async_generator);
+    function print_generator(output) {
+        output.print("function*");
+        print_lambda(this, output);
+    }
+    DEFPRINT(AST_GeneratorDefun, print_generator);
+    DEFPRINT(AST_GeneratorFunction, print_generator);
 
     /* -----[ jumps ]----- */
     function print_jump(kind, prop) {
@@ -1360,6 +1374,13 @@ function OutputStream(options) {
         output.space();
         this.expression.print(output);
     });
+    DEFPRINT(AST_Yield, function(output) {
+        output.print(this.nested ? "yield*" : "yield");
+        if (this.expression) {
+            output.space();
+            this.expression.print(output);
+        }
+    });
 
     /* -----[ literals ]----- */
     DEFPRINT(AST_Array, function(output) {
diff --git a/lib/parse.js b/lib/parse.js
index bc72550f..d5098ff5 100644
--- a/lib/parse.js
+++ b/lib/parse.js
@@ -112,13 +112,18 @@ var OPERATORS = makePredicate([
 
 var NEWLINE_CHARS = "\n\r\u2028\u2029";
 var OPERATOR_CHARS = "+-*&%=<>!?|~^";
-var PUNC_BEFORE_EXPRESSION = "[{(,;:";
-var PUNC_CHARS = PUNC_BEFORE_EXPRESSION + "`)}]";
+var PUNC_OPENERS = "[{(";
+var PUNC_SEPARATORS = ",;:";
+var PUNC_CLOSERS = ")}]";
+var PUNC_AFTER_EXPRESSION = PUNC_SEPARATORS + PUNC_CLOSERS;
+var PUNC_BEFORE_EXPRESSION = PUNC_OPENERS + PUNC_SEPARATORS;
+var PUNC_CHARS = PUNC_BEFORE_EXPRESSION + "`" + PUNC_CLOSERS;
 var WHITESPACE_CHARS = NEWLINE_CHARS + " \u00a0\t\f\u000b\u200b\u2000\u2001\u2002\u2003\u2004\u2005\u2006\u2007\u2008\u2009\u200a\u202f\u205f\u3000\uFEFF";
 var NON_IDENTIFIER_CHARS = makePredicate(characters("./'\"" + OPERATOR_CHARS + PUNC_CHARS + WHITESPACE_CHARS));
 
 NEWLINE_CHARS = makePredicate(characters(NEWLINE_CHARS));
 OPERATOR_CHARS = makePredicate(characters(OPERATOR_CHARS));
+PUNC_AFTER_EXPRESSION = makePredicate(characters(PUNC_AFTER_EXPRESSION));
 PUNC_BEFORE_EXPRESSION = makePredicate(characters(PUNC_BEFORE_EXPRESSION));
 PUNC_CHARS = makePredicate(characters(PUNC_CHARS));
 WHITESPACE_CHARS = makePredicate(characters(WHITESPACE_CHARS));
@@ -692,6 +697,7 @@ function parse($TEXT, options) {
         in_directives : true,
         in_funarg     : -1,
         in_function   : 0,
+        in_generator  : false,
         in_loop       : 0,
         labels        : [],
         peeked        : null,
@@ -829,12 +835,17 @@ function parse($TEXT, options) {
                 if (is_token(peek(), "keyword", "function")) {
                     next();
                     next();
-                    return function_(AST_AsyncDefun);
+                    if (!is("operator", "*")) return function_(AST_AsyncDefun);
+                    next();
+                    return function_(AST_AsyncGeneratorDefun);
                 }
                 break;
               case "await":
                 if (S.in_async) return simple_statement();
                 break;
+              case "yield":
+                if (S.in_generator) return simple_statement();
+                break;
             }
             return is_token(peek(), "punc", ":")
                 ? labeled_statement()
@@ -905,7 +916,9 @@ function parse($TEXT, options) {
 
               case "function":
                 next();
-                return function_(AST_Defun);
+                if (!is("operator", "*")) return function_(AST_Defun);
+                next();
+                return function_(AST_GeneratorDefun);
 
               case "if":
                 next();
@@ -1181,15 +1194,15 @@ function parse($TEXT, options) {
 
     var function_ = function(ctor) {
         var was_async = S.in_async;
+        var was_gen = S.in_generator;
         var name;
-        if (ctor === AST_AsyncDefun) {
+        if (/Defun$/.test(ctor.TYPE)) {
             name = as_symbol(AST_SymbolDefun);
-            S.in_async = true;
-        } else if (ctor === AST_Defun) {
-            name = as_symbol(AST_SymbolDefun);
-            S.in_async = false;
+            S.in_async = /^Async/.test(ctor.TYPE);
+            S.in_generator = /Generator/.test(ctor.TYPE);
         } else {
-            S.in_async = ctor === AST_AsyncFunction;
+            S.in_async = /^Async/.test(ctor.TYPE);
+            S.in_generator = /Generator/.test(ctor.TYPE);
             name = as_symbol(AST_SymbolLambda, true);
         }
         if (name && ctor !== AST_Accessor && !(name instanceof AST_SymbolDeclaration))
@@ -1218,6 +1231,7 @@ function parse($TEXT, options) {
         --S.in_function;
         S.in_loop = loop;
         S.labels = labels;
+        S.in_generator = was_gen;
         S.in_async = was_async;
         return new ctor({
             name: name,
@@ -1481,7 +1495,13 @@ function parse($TEXT, options) {
         }
         if (is("keyword", "function")) {
             next();
-            var func = function_(AST_Function);
+            var func;
+            if (is("operator", "*")) {
+                next();
+                func = function_(AST_GeneratorFunction);
+            } else {
+                func = function_(AST_Function);
+            }
             func.start = start;
             func.end = prev();
             return subscripts(func, allow_calls);
@@ -1492,7 +1512,13 @@ function parse($TEXT, options) {
             if (sym.name == "async") {
                 if (is("keyword", "function")) {
                     next();
-                    var func = function_(AST_AsyncFunction);
+                    var func;
+                    if (is("operator", "*")) {
+                        next();
+                        func = function_(AST_AsyncGeneratorFunction);
+                    } else {
+                        func = function_(AST_AsyncFunction);
+                    }
                     func.start = start;
                     func.end = prev();
                     return subscripts(func, allow_calls);
@@ -1567,6 +1593,21 @@ function parse($TEXT, options) {
             // allow trailing comma
             if (!options.strict && is("punc", "}")) break;
             var start = S.token;
+            if (is("operator", "*")) {
+                next();
+                var key = as_property_key();
+                var gen_start = S.token;
+                var gen = function_(AST_GeneratorFunction);
+                gen.start = gen_start;
+                gen.end = prev();
+                a.push(new AST_ObjectKeyVal({
+                    start: start,
+                    key: key,
+                    value: gen,
+                    end: prev(),
+                }));
+                continue;
+            }
             if (is("operator", "...")) {
                 next();
                 a.push(new AST_Spread({
@@ -1628,9 +1669,10 @@ function parse($TEXT, options) {
             }
             if (start.type == "name") switch (key) {
               case "async":
+                var is_gen = is("operator", "*") && next();
                 key = as_property_key();
                 var func_start = S.token;
-                var func = function_(AST_AsyncFunction);
+                var func = function_(is_gen ? AST_AsyncGeneratorFunction : AST_AsyncFunction);
                 func.start = func_start;
                 func.end = prev();
                 a.push(new AST_ObjectKeyVal({
@@ -1694,6 +1736,7 @@ function parse($TEXT, options) {
     function _make_symbol(type, token) {
         var name = token.value;
         if (name === "await" && S.in_async) unexpected(token);
+        if (name === "yield" && S.in_generator) unexpected(token);
         return new (name === "this" ? AST_This : type)({
             name: "" + name,
             start: token,
@@ -1870,12 +1913,42 @@ function parse($TEXT, options) {
         return expr;
     };
 
-    function maybe_unary() {
+    function maybe_unary(no_in) {
         var start = S.token;
+        if (S.in_async && is("name", "await")) {
+            if (S.in_funarg === S.in_function) croak("Invalid use of await in function argument");
+            S.input.context().regex_allowed = true;
+            next();
+            return new AST_Await({
+                start: start,
+                expression: maybe_unary(no_in),
+                end: prev(),
+            });
+        }
+        if (S.in_generator && is("name", "yield")) {
+            if (S.in_funarg === S.in_function) croak("Invalid use of yield in function argument");
+            S.input.context().regex_allowed = true;
+            next();
+            var exp = null;
+            var nested = false;
+            if (is("operator", "*")) {
+                next();
+                exp = maybe_assign(no_in);
+                nested = true;
+            } else if (is("punc") ? !PUNC_AFTER_EXPRESSION[S.token.value] : !can_insert_semicolon()) {
+                exp = maybe_assign(no_in);
+            }
+            return new AST_Yield({
+                start: start,
+                expression: exp,
+                nested: nested,
+                end: prev(),
+            });
+        }
         if (is("operator") && UNARY_PREFIX[start.value]) {
             next();
             handle_regexp();
-            var ex = make_unary(AST_UnaryPrefix, start, maybe_await());
+            var ex = make_unary(AST_UnaryPrefix, start, maybe_unary(no_in));
             ex.start = start;
             ex.end = prev();
             return ex;
@@ -1906,26 +1979,13 @@ function parse($TEXT, options) {
         return new ctor({ operator: op, expression: expr });
     }
 
-    function maybe_await() {
-        var start = S.token;
-        if (!(S.in_async && is("name", "await"))) return maybe_unary();
-        if (S.in_funarg === S.in_function) croak("Invalid use of await in function argument");
-        S.input.context().regex_allowed = true;
-        next();
-        return new AST_Await({
-            start: start,
-            expression: maybe_await(),
-            end: prev(),
-        });
-    }
-
     var expr_op = function(left, min_prec, no_in) {
         var op = is("operator") ? S.token.value : null;
         if (op == "in" && no_in) op = null;
         var prec = op != null ? PRECEDENCE[op] : null;
         if (prec != null && prec > min_prec) {
             next();
-            var right = expr_op(maybe_await(), op == "**" ? prec - 1 : prec, no_in);
+            var right = expr_op(maybe_unary(no_in), op == "**" ? prec - 1 : prec, no_in);
             return expr_op(new AST_Binary({
                 start    : left.start,
                 left     : left,
@@ -1938,7 +1998,7 @@ function parse($TEXT, options) {
     };
 
     function expr_ops(no_in) {
-        return expr_op(maybe_await(), 0, no_in);
+        return expr_op(maybe_unary(no_in), 0, no_in);
     }
 
     var maybe_conditional = function(no_in) {
diff --git a/lib/scope.js b/lib/scope.js
index 5f10e33d..fb51486a 100644
--- a/lib/scope.js
+++ b/lib/scope.js
@@ -121,7 +121,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options) {
     var next_def_id = 0;
     var scope = self.parent_scope = null;
     var tw = new TreeWalker(function(node, descend) {
-        if (is_defun(node)) {
+        if (node instanceof AST_LambdaDefinition) {
             node.name.walk(tw);
             walk_scope(function() {
                 node.argnames.forEach(function(argname) {
@@ -439,7 +439,7 @@ AST_BlockScope.DEFMETHOD("find_variable", function(name) {
 
 AST_BlockScope.DEFMETHOD("def_function", function(symbol, init) {
     var def = this.def_variable(symbol, init);
-    if (!def.init || is_defun(def.init)) def.init = init;
+    if (!def.init || def.init instanceof AST_LambdaDefinition) def.init = init;
     this.functions.set(symbol.name, def);
     return def;
 });
@@ -448,7 +448,7 @@ AST_BlockScope.DEFMETHOD("def_variable", function(symbol, init) {
     var def = this.variables.get(symbol.name);
     if (def) {
         def.orig.push(symbol);
-        if (is_function(def.init)) def.init = init;
+        if (def.init instanceof AST_LambdaExpression) def.init = init;
     } else {
         def = this.make_def(symbol, init);
         this.variables.set(symbol.name, def);
diff --git a/lib/transform.js b/lib/transform.js
index b84bf0b9..ede6e254 100644
--- a/lib/transform.js
+++ b/lib/transform.js
@@ -157,6 +157,9 @@ TreeTransformer.prototype = new TreeWalker;
     DEF(AST_Await, function(self, tw) {
         self.expression = self.expression.transform(tw);
     });
+    DEF(AST_Yield, function(self, tw) {
+        if (self.expression) self.expression = self.expression.transform(tw);
+    });
     DEF(AST_Dot, function(self, tw) {
         self.expression = self.expression.transform(tw);
     });
diff --git a/test/compress/yields.js b/test/compress/yields.js
new file mode 100644
index 00000000..2c06a4ac
--- /dev/null
+++ b/test/compress/yields.js
@@ -0,0 +1,589 @@
+binary: {
+    input: {
+        var a = function*() {
+            console.log(6 * (yield "PA" + "SS"));
+        }();
+        console.log(a.next("FAIL").value);
+        console.log(a.next(7).done);
+    }
+    expect_exact: 'var a=function*(){console.log(6*(yield"PA"+"SS"))}();console.log(a.next("FAIL").value);console.log(a.next(7).done);'
+    expect_stdout: [
+        "PASS",
+        "42",
+        "true",
+    ]
+    node_version: ">=4"
+}
+
+empty_yield: {
+    input: {
+        var a = function*() {
+            yield;
+            console.log(yield);
+            yield
+            "FAIL 1";
+        }();
+        console.log(a.next("FAIL 2").value);
+        console.log(a.next("FAIL 3").value);
+        console.log(a.next("PASS").value);
+        console.log(a.next("FAIL 4").done);
+    }
+    expect_exact: 'var a=function*(){yield;console.log(yield);yield;"FAIL 1"}();console.log(a.next("FAIL 2").value);console.log(a.next("FAIL 3").value);console.log(a.next("PASS").value);console.log(a.next("FAIL 4").done);'
+    expect_stdout: [
+        "undefined",
+        "undefined",
+        "PASS",
+        "undefined",
+        "true",
+    ]
+    node_version: ">=4"
+}
+
+empty_yield_conditional: {
+    input: {
+        var a = function*() {
+            console.log((yield) ? yield : yield);
+        }();
+        console.log(a.next("FAIL 1").value);
+        console.log(a.next("FAIL 2").value);
+        console.log(a.next("PASS").value);
+        console.log(a.next("FAIL 3").done);
+    }
+    expect_exact: 'var a=function*(){console.log((yield)?yield:yield)}();console.log(a.next("FAIL 1").value);console.log(a.next("FAIL 2").value);console.log(a.next("PASS").value);console.log(a.next("FAIL 3").done);'
+    expect_stdout: [
+        "undefined",
+        "undefined",
+        "PASS",
+        "undefined",
+        "true",
+    ]
+    node_version: ">=4"
+}
+
+nested_yield: {
+    input: {
+        console.log(function*() {
+            (yield*
+            f())
+            function* f() {
+                return "FAIL";
+            }
+            yield*
+            f();
+            yield *f();
+        }().next().value || "PASS");
+    }
+    expect_exact: 'console.log(function*(){yield*f();function*f(){return"FAIL"}yield*f();yield*f()}().next().value||"PASS");'
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+pause_resume: {
+    input: {
+        function* f() {
+            console.log(yield "PASS");
+        }
+        var a = f();
+        console.log(a.next("FAIL").value);
+        console.log(a.next(42).done);
+    }
+    expect_exact: 'function*f(){console.log(yield"PASS")}var a=f();console.log(a.next("FAIL").value);console.log(a.next(42).done);'
+    expect_stdout: [
+        "PASS",
+        "42",
+        "true",
+    ]
+    node_version: ">=4"
+}
+
+collapse_vars_1: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function*() {
+            a = "PASS";
+            yield 42;
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function*() {
+            a = "PASS";
+            yield 42;
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+collapse_vars_2: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function*() {
+            yield (a = "PASS");
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function*() {
+            yield (a = "PASS");
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+collapse_vars_3: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = "FAIL";
+        (function*() {
+            yield (a = "PASS", 42);
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect: {
+        var a = "FAIL";
+        (function*() {
+            yield (a = "PASS", 42);
+            return "PASS";
+        })().next();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+collapse_vars_4: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        var a = "FAIL";
+        var b = function*(c) {
+            return c;
+        }(a = "PASS");
+        console.log(a, b.next().done);
+    }
+    expect: {
+        var a = "FAIL";
+        var b = function*(c) {
+            return c;
+        }(a = "PASS");
+        console.log(a, b.next().done);
+    }
+    expect_stdout: "PASS true"
+    node_version: ">=4"
+}
+
+collapse_property_lambda: {
+    options = {
+        collapse_vars: true,
+        pure_getters: "strict",
+    }
+    input: {
+        console.log(function* f() {
+            f.g = () => 42;
+            return f.g();
+        }().next().value);
+    }
+    expect: {
+        console.log(function* f() {
+            return (f.g = () => 42)();
+        }().next().value);
+    }
+    expect_stdout: "42"
+    node_version: ">=4"
+}
+
+evaluate: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = function*() {}();
+        console.log(typeof a);
+    }
+    expect: {
+        var a = function*() {}();
+        console.log(typeof a);
+    }
+    expect_stdout: "object"
+    node_version: ">=4"
+}
+
+functions: {
+    options = {
+        functions: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        !function*() {
+            var a = function* a() {
+                return a && "a";
+            };
+            var b = function* x() {
+                return !!x;
+            };
+            var c = function*(c) {
+                return c;
+            };
+            if (yield* c(yield* b(yield* a()))) {
+                var d = function*() {};
+                var e = function* y() {
+                    return typeof y;
+                };
+                var f = function*(f) {
+                    return f;
+                };
+                console.log(yield* a(yield* d()), yield* b(yield* e()), yield* c(yield* f(42)), typeof d, yield* e(), typeof f);
+            }
+        }().next();
+    }
+    expect: {
+        !function*() {
+            function* a() {
+                return a && "a";
+            }
+            function* b() {
+                return !!b;
+            }
+            var c = function*(c) {
+                return c;
+            };
+            if (yield* c(yield* b(yield* a()))) {
+                function* d() {}
+                function* e() {
+                    return typeof e;
+                }
+                var f = function*(f) {
+                    return f;
+                };
+                console.log(yield* a(yield* d()), yield* b(yield* e()), yield* c(yield* f(42)), typeof d, yield* e(), typeof f);
+            }
+        }().next();
+    }
+    expect_stdout: "a true 42 function function function"
+    node_version: ">=4"
+}
+
+functions_use_strict: {
+    options = {
+        functions: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        "use strict";
+        !function*() {
+            var a = function* a() {
+                return a && "a";
+            };
+            var b = function* x() {
+                return !!x;
+            };
+            var c = function*(c) {
+                return c;
+            };
+            if (yield* c(yield* b(yield* a()))) {
+                var d = function*() {};
+                var e = function* y() {
+                    return typeof y;
+                };
+                var f = function*(f) {
+                    return f;
+                };
+                console.log(yield* a(yield* d()), yield* b(yield* e()), yield* c(yield* f(42)), typeof d, yield* e(), typeof f);
+            }
+        }().next();
+    }
+    expect: {
+        "use strict";
+        !function*() {
+            function* a() {
+                return a && "a";
+            }
+            function* b() {
+                return !!b;
+            }
+            var c = function*(c) {
+                return c;
+            };
+            if (yield* c(yield* b(yield* a()))) {
+                var d = function*() {};
+                var e = function* y() {
+                    return typeof y;
+                };
+                var f = function*(f) {
+                    return f;
+                };
+                console.log(yield* a(yield* d()), yield* b(yield* e()), yield* c(yield* f(42)), typeof d, yield* e(), typeof f);
+            }
+        }().next();
+    }
+    expect_stdout: "a true 42 function function function"
+    node_version: ">=4"
+}
+
+negate_iife: {
+    options = {
+        negate_iife: true,
+        side_effects: true,
+    }
+    input: {
+        (function*(a) {
+            console.log(a);
+        })("PASS").next();
+    }
+    expect: {
+        !function*(a) {
+            console.log(a);
+        }("PASS").next();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+reduce_iife_1: {
+    options = {
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(function*(a) {
+            yield a;
+        }(42).next().value);
+    }
+    expect: {
+        console.log(function*(a) {
+            yield 42;
+        }().next().value);
+    }
+    expect_stdout: "42"
+    node_version: ">=4"
+}
+
+reduce_iife_2: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = "PASS";
+        (function*() {
+            a = "FAIL";
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        (function*() {
+            a = "FAIL";
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+reduce_single_use_defun: {
+    options = {
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        function* f(a) {
+            console.log(a);
+        }
+        f("PASS").next();
+    }
+    expect: {
+        (function*(a) {
+            console.log(a);
+        })("PASS").next();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+reduce_tagged: {
+    options = {
+        reduce_funcs: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        function* f() {
+            function g() {
+                h`foo`;
+            }
+            g();
+            function h(s) {
+                console.log(s[0]);
+            }
+            h([ "bar" ]);
+        }
+        f().next();
+    }
+    expect: {
+        function* f() {
+            (function() {
+                h`foo`;
+            })();
+            function h(s) {
+                console.log(s[0]);
+            }
+            h([ "bar" ]);
+        }
+        f().next();
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=4"
+}
+
+reduce_tagged_async: {
+    options = {
+        reduce_funcs: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        async function* f() {
+            function g() {
+                h`foo`;
+            }
+            g();
+            function h(s) {
+                console.log(s[0]);
+            }
+            h([ "bar" ]);
+        }
+        f().next();
+    }
+    expect: {
+        async function* f() {
+            (function() {
+                h`foo`;
+            })();
+            function h(s) {
+                console.log(s[0]);
+            }
+            h([ "bar" ]);
+        }
+        f().next();
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+    ]
+    node_version: ">=10"
+}
+
+lift_sequence: {
+    options = {
+        sequences: true,
+        yields: true,
+    }
+    input: {
+        console.log(function*() {
+            yield (console, "PASS");
+        }().next().value);
+    }
+    expect: {
+        console.log(function*() {
+            console, yield "PASS";
+        }().next().value);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}
+
+inline_nested_yield: {
+    options = {
+        inline: true,
+        sequences: true,
+        yields: true,
+    }
+    input: {
+        var a = function*() {
+            yield* function*() {
+                yield "foo";
+                return "FAIL";
+            }();
+        }(), b;
+        do {
+            b = a.next();
+            console.log(b.value);
+        } while (!b.done);
+    }
+    expect: {
+        var a = function*() {
+            yield "foo",
+            "FAIL";
+        }(), b;
+        do {
+            b = a.next(),
+            console.log(b.value);
+        } while (!b.done);
+    }
+    expect_stdout: [
+        "foo",
+        "undefined",
+    ]
+    node_version: ">=4"
+}
+
+issue_4618: {
+    options = {
+        functions: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        console.log(typeof function() {
+            var yield = function* f() {
+                console || f();
+            };
+            console.log;
+            return yield;
+        }());
+    }
+    expect: {
+        console.log(typeof function() {
+            var yield = function* f() {
+                console || f();
+            };
+            console.log;
+            return yield;
+        }());
+    }
+    expect_stdout: "function"
+    node_version: ">=4"
+}
diff --git a/test/mocha/async.js b/test/mocha/awaits.js
index d4e96609..d4e96609 100644
--- a/test/mocha/async.js
+++ b/test/mocha/awaits.js
diff --git a/test/mocha/yields.js b/test/mocha/yields.js
new file mode 100644
index 00000000..d995d61d
--- /dev/null
+++ b/test/mocha/yields.js
@@ -0,0 +1,82 @@
+var assert = require("assert");
+var UglifyJS = require("../node");
+
+describe("generator", function() {
+    it("Should reject `yield` as symbol name within generator functions only", function() {
+        [
+            "function yield() {}",
+            "function(yield) {}",
+            "function() { yield:{} }",
+            "function() { var yield; }",
+            "function() { function yield() {} }",
+            "function() { try {} catch (yield) {} }",
+        ].forEach(function(code) {
+            var ast = UglifyJS.parse("(" + code + ")();");
+            assert.strictEqual(ast.TYPE, "Toplevel");
+            assert.strictEqual(ast.body.length, 1);
+            assert.strictEqual(ast.body[0].TYPE, "SimpleStatement");
+            assert.strictEqual(ast.body[0].body.TYPE, "Call");
+            assert.strictEqual(ast.body[0].body.expression.TYPE, "Function");
+            assert.throws(function() {
+                UglifyJS.parse("(" + code.replace(/^function/, "function*") + ")();");
+            }, function(e) {
+                return e instanceof UglifyJS.JS_Parse_Error;
+            }, code);
+        });
+    });
+    it("Should reject `yield` expression outside of generator functions", function() {
+        [
+            "yield 42;",
+            "function f() { yield 42; }",
+            "function* f() { function g() { yield 42; } }",
+        ].forEach(function(code) {
+            assert.throws(function() {
+                UglifyJS.parse(code);
+            }, function(e) {
+                return e instanceof UglifyJS.JS_Parse_Error;
+            }, code);
+        });
+    });
+    it("Should reject `yield` expression directly on computed key of function argument", function() {
+        [
+            "function f({ [yield 42]: a }) {}",
+            "function* f({ [yield 42]: a }) {}",
+        ].forEach(function(code) {
+            assert.throws(function() {
+                UglifyJS.parse(code);
+            }, function(e) {
+                return e instanceof UglifyJS.JS_Parse_Error;
+            }, code);
+        });
+    });
+    it("Should accept `yield` expression nested within computed key of function argument", function() {
+        [
+            "function f({ [function*() { yield 42; }()]: a }) {}",
+            "function* f({ [function*() { yield 42; }()]: a }) {}",
+        ].forEach(function(code) {
+            var ast = UglifyJS.parse(code);
+            assert.strictEqual(ast.TYPE, "Toplevel");
+            assert.strictEqual(ast.body.length, 1);
+            assert.strictEqual(ast.body[0].argnames.length, 1);
+            assert.strictEqual(ast.body[0].argnames[0].TYPE, "DestructuredObject");
+        });
+    });
+    it("Should reject `yield*` without an expression", function() {
+        [
+            "yield*",
+            "yield*;",
+            "yield*,",
+            "(yield*)",
+            "[ yield* ]",
+            "42[yield*]",
+            "yield* && 42",
+        ].forEach(function(code) {
+            code = "function* f() { " + code + " }";
+            assert.throws(function() {
+                UglifyJS.parse(code);
+            }, function(e) {
+                return e instanceof UglifyJS.JS_Parse_Error;
+            }, code);
+        });
+    });
+});
diff --git a/test/reduce.js b/test/reduce.js
index 9d5f053c..5f192aca 100644
--- a/test/reduce.js
+++ b/test/reduce.js
@@ -666,11 +666,7 @@ function is_timed_out(result) {
 }
 
 function is_statement(node) {
-    return node instanceof U.AST_Statement
-        && !(node instanceof U.AST_Arrow
-            || node instanceof U.AST_AsyncArrow
-            || node instanceof U.AST_AsyncFunction
-            || node instanceof U.AST_Function);
+    return node instanceof U.AST_Statement && !(node instanceof U.AST_LambdaExpression);
 }
 
 function merge_sequence(array, node) {
diff --git a/test/ufuzz/index.js b/test/ufuzz/index.js
index 5256ec63..58228bdb 100644
--- a/test/ufuzz/index.js
+++ b/test/ufuzz/index.js
@@ -134,6 +134,7 @@ var SUPPORT = function(matrix) {
 }({
     arrow: "a => 0;",
     async: "async function f(){}",
+    async_generator: "async function* f(){}",
     bigint: "42n",
     catch_omit_var: "try {} catch {}",
     computed_key: "({[0]: 0});",
@@ -141,6 +142,7 @@ var SUPPORT = function(matrix) {
     default_value: "[ a = 0 ] = [];",
     destructuring: "[] = [];",
     exponentiation: "0 ** 0",
+    generator: "function* f(){}",
     let: "let a;",
     rest: "var [...a] = [];",
     rest_object: "var {...a} = {};",
@@ -349,6 +351,7 @@ var avoid_vars = [];
 var block_vars = [];
 var unique_vars = [];
 var async = false;
+var generator = false;
 var loops = 0;
 var funcs = 0;
 var called = Object.create(null);
@@ -368,6 +371,7 @@ function createTopLevelCode() {
     block_vars.length = 0;
     unique_vars.length = 0;
     async = false;
+    generator = false;
     loops = 0;
     funcs = 0;
     called = Object.create(null);
@@ -395,9 +399,11 @@ function addTrailingComma(list) {
     return SUPPORT.trailing_comma && list && rng(20) == 0 ? list + "," : list;
 }
 
-function createParams(was_async, noDuplicate) {
+function createParams(was_async, was_generator, noDuplicate) {
     var save_async = async;
     if (was_async) async = true;
+    var save_generator = generator;
+    if (was_generator) generator = true;
     var len = unique_vars.length;
     var params = [];
     for (var n = rng(4); --n >= 0;) {
@@ -406,6 +412,7 @@ function createParams(was_async, noDuplicate) {
         params.push(name);
     }
     unique_vars.length = len;
+    generator = save_generator;
     async = save_async;
     return addTrailingComma(params.join(", "));
 }
@@ -434,7 +441,7 @@ function createArgs(recurmax, stmtDepth, canThrow, noTemplate) {
     return "(" + addTrailingComma(args.join(", ")) + ")";
 }
 
-function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was_async) {
+function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was_async, was_generator) {
     var avoid = [];
     var len = unique_vars.length;
     var pairs = createPairs(recurmax, !nameLenBefore);
@@ -476,9 +483,18 @@ function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was
             async = false;
             if (save_async || was_async) addAvoidVar("await");
         }
+        var save_generator = generator;
+        if (was_generator != null) {
+            generator = false;
+            if (save_generator || was_generator) addAvoidVar("yield");
+        }
         avoid.forEach(addAvoidVar);
         var save_vars = nameLenBefore && VAR_NAMES.splice(nameLenBefore);
         if (nameFn) nameFn();
+        if (was_generator != null) {
+            generator = was_generator;
+            if (save_generator || was_generator) removeAvoidVar("yield");
+        }
         if (was_async != null) {
             async = was_async;
             if (save_async || was_async) removeAvoidVar("await");
@@ -486,6 +502,7 @@ function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was
         if (valueFn) valueFn();
         if (save_vars) [].push.apply(VAR_NAMES, save_vars);
         avoid.forEach(removeAvoidVar);
+        generator = save_generator;
         async = save_async;
     }
 
@@ -509,7 +526,10 @@ function createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, was
         unique_vars.push("a", "b", "c", "undefined", "NaN", "Infinity");
         var save_async = async;
         if (was_async) async = true;
+        var save_generator = generator;
+        if (was_generator) generator = true;
         var name = createVarName(MANDATORY);
+        generator = save_generator;
         async = save_async;
         unique_vars.length -= 6;
         avoid.push(name);
@@ -707,7 +727,25 @@ function mayCreateBlockVariables(recurmax, stmtDepth, canThrow, fn) {
 }
 
 function makeFunction(name) {
-    return (async ? "async function " : "function ") + name;
+    if (generator) {
+        name = "function* " + name;
+    } else {
+        name = "function " + name;
+    }
+    if (async) name = "async " + name;
+    return name;
+}
+
+function invokeGenerator(was_generator) {
+    if (generator && !was_generator) switch (rng(4)) {
+      case 0:
+        return ".next()";
+      case 1:
+        return ".next().done";
+      case 2:
+        return ".next().value";
+    }
+    return "";
 }
 
 function createFunction(recurmax, allowDefun, canThrow, stmtDepth) {
@@ -718,6 +756,15 @@ function createFunction(recurmax, allowDefun, canThrow, stmtDepth) {
     var nameLenBefore = VAR_NAMES.length;
     var save_async = async;
     async = SUPPORT.async && rng(50) == 0;
+    var save_generator = generator;
+    generator = SUPPORT.generator && rng(50) == 0;
+    if (async && generator && !SUPPORT.async_generator) {
+        if (rng(2)) {
+            async = false;
+        } else {
+            generator = false;
+        }
+    }
     createBlockVariables(recurmax, stmtDepth, canThrow, function(defns) {
         if (allowDefun || rng(5) > 0) {
             name = "f" + funcs++;
@@ -729,12 +776,12 @@ function createFunction(recurmax, allowDefun, canThrow, stmtDepth) {
         var params;
         if (SUPPORT.destructuring && (!allowDefun || !(name in called)) && rng(2)) {
             called[name] = false;
-            var pairs = createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, save_async);
+            var pairs = createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, save_async, save_generator);
             params = pairs.names.join(", ");
             if (!pairs.has_rest) params = addTrailingComma(params);
             args = "(" + addTrailingComma(pairs.values.join(", ")) + ")";
         } else {
-            params = createParams(save_async);
+            params = createParams(save_async, save_generator);
         }
         s.push(makeFunction(name) + "(" + params + "){", strictMode());
         s.push(defns());
@@ -748,6 +795,8 @@ function createFunction(recurmax, allowDefun, canThrow, stmtDepth) {
         s.push("}", "");
         s = filterDirective(s).join("\n");
     });
+    var call_next = invokeGenerator(save_generator);
+    generator = save_generator;
     async = save_async;
     VAR_NAMES.length = nameLenBefore;
 
@@ -755,9 +804,11 @@ function createFunction(recurmax, allowDefun, canThrow, stmtDepth) {
         // avoid "function statements" (decl inside statements)
         s = "var " + createVarName(MANDATORY) + " = " + s;
         s += args || createArgs(recurmax, stmtDepth, canThrow);
+        s += call_next;
     } else if (!(name in called) || args || rng(3)) {
         s += "var " + createVarName(MANDATORY) + " = " + name;
         s += args || createArgs(recurmax, stmtDepth, canThrow);
+        s += call_next;
     }
 
     return s + ";";
@@ -1019,7 +1070,9 @@ function createExpression(recurmax, noComma, stmtDepth, canThrow) {
         return "((c = c + 1) + (" + _createExpression(recurmax, noComma, stmtDepth, canThrow) + "))"; // c only gets incremented
       default:
         var expr = "(" + _createExpression(recurmax, noComma, stmtDepth, canThrow) + ")";
-        return async && rng(50) == 0 ? "(await" + expr + ")" : expr;
+        if (async && rng(50) == 0) return "(await" + expr + ")";
+        if (generator && rng(50) == 0) return "(yield" + (canThrow && rng(20) == 0 ? "*" : "") + expr + ")";
+        return expr;
     }
 }
 
@@ -1027,6 +1080,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
     var p = 0;
     switch (rng(_createExpression.N)) {
       case p++:
+        if (generator && rng(50) == 0) return "yield";
       case p++:
         return createUnaryPrefix() + (rng(2) ? "a" : "b");
       case p++:
@@ -1082,25 +1136,34 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
         var nameLenBefore = VAR_NAMES.length;
         var save_async = async;
         async = SUPPORT.async && rng(50) == 0;
+        var save_generator = generator;
+        generator = SUPPORT.generator && rng(50) == 0;
+        if (async && generator && !SUPPORT.async_generator) {
+            if (rng(2)) {
+                async = false;
+            } else {
+                generator = false;
+            }
+        }
         unique_vars.push("c");
         var name = createVarName(MAYBE); // note: this name is only accessible from _within_ the function. and immutable at that.
         unique_vars.pop();
         var s = [];
         switch (rng(5)) {
           case 0:
-            if (SUPPORT.arrow && !name && rng(2)) {
+            if (SUPPORT.arrow && !name && !generator && rng(2)) {
                 var args, suffix;
                 (rng(2) ? createBlockVariables : function() {
                     arguments[3]();
                 })(recurmax, stmtDepth, canThrow, function(defns) {
                     var params;
                     if (SUPPORT.destructuring && rng(2)) {
-                        var pairs = createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, save_async);
+                        var pairs = createAssignmentPairs(recurmax, stmtDepth, canThrow, nameLenBefore, save_async, save_generator);
                         params = pairs.names.join(", ");
                         if (!pairs.has_rest) params = addTrailingComma(params);
                         args = "(" + addTrailingComma(pairs.values.join(", ")) + ")";
                     } else {
-                        params = createParams(save_async, NO_DUPLICATE);
+                        params = createParams(save_async, save_generator, NO_DUPLICATE);
                     }
                     params = (async ? "async (" : "(") + params + ") => ";
                     if (defns) {
@@ -1127,6 +1190,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                         suffix = ")";
                     }
                 });
+                generator = save_generator;
                 async = save_async;
                 VAR_NAMES.length = nameLenBefore;
                 if (!args && rng(2)) args = createArgs(recurmax, stmtDepth, canThrow);
@@ -1137,7 +1201,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                     "(" + makeFunction(name) + "(){",
                     strictMode(),
                     createStatements(rng(5) + 1, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth),
-                    rng(2) ? "})" : "})()"
+                    rng(2) ? "})" : "})()" + invokeGenerator(save_generator)
                 );
             }
             break;
@@ -1146,7 +1210,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                 "+" + makeFunction(name) + "(){",
                 strictMode(),
                 createStatements(rng(5) + 1, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth),
-                "}()"
+                "}()" + invokeGenerator(save_generator)
             );
             break;
           case 2:
@@ -1154,7 +1218,7 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                 "!" + makeFunction(name) + "(){",
                 strictMode(),
                 createStatements(rng(5) + 1, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth),
-                "}()"
+                "}()" + invokeGenerator(save_generator)
             );
             break;
           case 3:
@@ -1162,15 +1226,16 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                 "void " + makeFunction(name) + "(){",
                 strictMode(),
                 createStatements(rng(5) + 1, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth),
-                "}()"
+                "}()" + invokeGenerator(save_generator)
             );
             break;
           default:
             async = false;
+            generator = false;
             var instantiate = rng(4) ? "new " : "";
             createBlockVariables(recurmax, stmtDepth, canThrow, function(defns) {
                 s.push(
-                    instantiate + "function " + name + "(" + createParams(save_async) + "){",
+                    instantiate + "function " + name + "(" + createParams(save_async, save_generator) + "){",
                     strictMode(),
                     defns()
                 );
@@ -1180,11 +1245,13 @@ function _createExpression(recurmax, noComma, stmtDepth, canThrow) {
                 }
                 s.push(_createStatements(rng(5) + 1, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth));
             });
+            generator = save_generator;
             async = save_async;
             VAR_NAMES.length = nameLenBefore;
             s.push(rng(2) ? "}" : "}" + createArgs(recurmax, stmtDepth, canThrow, instantiate));
             break;
         }
+        generator = save_generator;
         async = save_async;
         VAR_NAMES.length = nameLenBefore;
         return filterDirective(s).join("\n");
@@ -1328,8 +1395,6 @@ function createTemplateLiteral(recurmax, stmtDepth, canThrow) {
 }
 
 var SAFE_KEYS = [
-    "length",
-    "foo",
     "a",
     "b",
     "c",
@@ -1337,7 +1402,13 @@ var SAFE_KEYS = [
     "null",
     "NaN",
     "Infinity",
+    "done",
+    "foo",
     "in",
+    "length",
+    "next",
+    "then",
+    "value",
     "var",
 ];
 var KEYS = [
@@ -1367,12 +1438,14 @@ function createObjectKey(recurmax, stmtDepth, canThrow) {
 function createObjectFunction(recurmax, stmtDepth, canThrow) {
     var nameLenBefore = VAR_NAMES.length;
     var save_async = async;
+    var save_generator = generator;
     var s;
     var name = createObjectKey(recurmax, stmtDepth, canThrow);
     var fn;
     switch (rng(SUPPORT.computed_key ? 3 : 2)) {
       case 0:
         async = false;
+        generator = false;
         fn = function(defns) {
             s = [
                 "get " + name + "(){",
@@ -1390,6 +1463,7 @@ function createObjectFunction(recurmax, stmtDepth, canThrow) {
             prop = getDotKey();
         } while (name == prop);
         async = false;
+        generator = false;
         fn = function(defns) {
             s = [
                 "set " + name + "(" + createVarName(MANDATORY) + "){",
@@ -1403,9 +1477,19 @@ function createObjectFunction(recurmax, stmtDepth, canThrow) {
         break;
       default:
         async = SUPPORT.async && rng(50) == 0;
+        generator = SUPPORT.generator && rng(50) == 0;
+        if (async && generator && !SUPPORT.async_generator) {
+            if (rng(2)) {
+                async = false;
+            } else {
+                generator = false;
+            }
+        }
         fn = function(defns) {
+            if (generator) name = "*" + name;
+            if (async) name = "async "+ name;
             s = [
-                (async ? "async " : "") + name + "(" + createParams(save_async, NO_DUPLICATE) + "){",
+                name + "(" + createParams(save_async, save_generator, NO_DUPLICATE) + "){",
                 strictMode(),
                 defns(),
                 _createStatements(3, recurmax, canThrow, CANNOT_BREAK, CANNOT_CONTINUE, CAN_RETURN, stmtDepth),
@@ -1415,6 +1499,7 @@ function createObjectFunction(recurmax, stmtDepth, canThrow) {
         break;
     }
     createBlockVariables(recurmax, stmtDepth, canThrow, fn);
+    generator = save_generator;
     async = save_async;
     VAR_NAMES.length = nameLenBefore;
     return filterDirective(s).join("\n");
@@ -1563,8 +1648,11 @@ function createTypeofExpr(recurmax, stmtDepth, canThrow) {
 
     function createVar() {
         var save_async = async;
+        var save_generator = generator;
         if (!async && avoid_vars.indexOf("await") >= 0) async = true;
+        if (!generator && avoid_vars.indexOf("yield") >= 0) generator = true;
         var name = createVarName(MANDATORY, DONT_STORE);
+        generator = save_generator;
         async = save_async;
         return name;
     }
@@ -1613,7 +1701,11 @@ function getVarName(noConst) {
     do {
         if (--tries < 0) return "a";
         name = VAR_NAMES[INITIAL_NAMES_LEN + rng(VAR_NAMES.length - INITIAL_NAMES_LEN)];
-    } while (!name || avoid_vars.indexOf(name) >= 0 || noConst && block_vars.indexOf(name) >= 0 || async && name == "await");
+    } while (!name
+        || avoid_vars.indexOf(name) >= 0
+        || noConst && block_vars.indexOf(name) >= 0
+        || async && name == "await"
+        || generator && name == "yield");
     return name;
 }
 
@@ -1625,7 +1717,10 @@ function createVarName(maybe, dontStore) {
             name = VAR_NAMES[rng(VAR_NAMES.length)];
             if (--tries < 0) suffix++;
             if (suffix) name += "_" + suffix;
-        } while (unique_vars.indexOf(name) >= 0 || block_vars.indexOf(name) >= 0 || async && name == "await");
+        } while (unique_vars.indexOf(name) >= 0
+            || block_vars.indexOf(name) >= 0
+            || async && name == "await"
+            || generator && name == "yield");
         if (!dontStore) VAR_NAMES.push(name);
         return name;
     }