enhance `arguments` (#3193)

fixes #3192

2 files changed, 52 insertions, 37 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 56c8fc47..0268e07f 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -518,14 +518,15 @@ merge(Compressor.prototype, {
             var sym = node.left;
             if (!(sym instanceof AST_SymbolRef)) return;
             var d = sym.definition();
+            var safe = safe_to_assign(tw, d, sym.scope, node.right);
+            d.assignments++;
+            if (!safe) return;
             var fixed = d.fixed;
             if (!fixed && node.operator != "=") return;
-            if (!safe_to_assign(tw, d, sym.scope, node.right)) return;
             var eq = node.operator == "=";
             var value = eq ? node.right : node;
             if (is_modified(compressor, tw, node, value, 0)) return;
             d.references.push(sym);
-            d.assignments++;
             if (!eq) d.chained = true;
             d.fixed = eq ? function() {
                 return node.right;
@@ -657,7 +658,7 @@ merge(Compressor.prototype, {
                 // So existing transformation rules can work on them.
                 node.argnames.forEach(function(arg, i) {
                     var d = arg.definition();
-                    if (!node.uses_arguments && d.fixed === undefined) {
+                    if (d.fixed === undefined && (!node.uses_arguments || tw.has_directive("use strict"))) {
                         d.fixed = function() {
                             return iife.args[i] || make_node(AST_Undefined, iife);
                         };
@@ -760,11 +761,12 @@ merge(Compressor.prototype, {
             var exp = node.expression;
             if (!(exp instanceof AST_SymbolRef)) return;
             var d = exp.definition();
+            var safe = safe_to_assign(tw, d, exp.scope, true);
+            d.assignments++;
+            if (!safe) return;
             var fixed = d.fixed;
             if (!fixed) return;
-            if (!safe_to_assign(tw, d, exp.scope, true)) return;
             d.references.push(exp);
-            d.assignments++;
             d.chained = true;
             d.fixed = function() {
                 return make_node(AST_Binary, node, {
@@ -3289,6 +3291,15 @@ merge(Compressor.prototype, {
         // this scope (not in nested scopes).
         var scope = this;
         var tw = new TreeWalker(function(node, descend) {
+            if (node instanceof AST_Lambda && node.uses_arguments && !tw.has_directive("use strict")) {
+                node.argnames.forEach(function(argname) {
+                    var def = argname.definition();
+                    if (!(def.id in in_use_ids)) {
+                        in_use_ids[def.id] = true;
+                        in_use.push(def);
+                    }
+                });
+            }
             if (node === self) return;
             if (node instanceof AST_Defun) {
                 var node_def = node.name.definition();
@@ -3376,8 +3387,7 @@ merge(Compressor.prototype, {
                     // any declarations with same name will overshadow
                     // name of this anonymous function and can therefore
                     // never be used anywhere
-                    if (!(def.id in in_use_ids) || def.orig.length > 1)
-                        node.name = null;
+                    if (!(def.id in in_use_ids) || def.orig.length > 1) node.name = null;
                 }
                 if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
                     var trim = !compressor.option("keep_fargs");
@@ -3389,8 +3399,7 @@ merge(Compressor.prototype, {
                                 a.pop();
                                 compressor[sym.unreferenced() ? "warn" : "info"]("Dropping unused function argument {name} [{file}:{line},{col}]", template(sym));
                             }
-                        }
-                        else {
+                        } else {
                             trim = false;
                         }
                     }
@@ -6208,6 +6217,39 @@ merge(Compressor.prototype, {
                 }
             }
         }
+        var fn;
+        if (compressor.option("arguments")
+            && expr instanceof AST_SymbolRef
+            && expr.name == "arguments"
+            && expr.definition().orig.length == 1
+            && (fn = expr.scope) instanceof AST_Lambda
+            && prop instanceof AST_Number) {
+            var index = prop.getValue();
+            var argname = fn.argnames[index];
+            if (argname && compressor.has_directive("use strict")) {
+                var def = argname.definition();
+                if (!compressor.option("reduce_vars") || def.assignments || def.orig.length > 1) {
+                    argname = null;
+                }
+            } else if (!argname && !compressor.option("keep_fargs") && index < fn.argnames.length + 5) {
+                while (index >= fn.argnames.length) {
+                    argname = make_node(AST_SymbolFunarg, fn, {
+                        name: fn.make_var_name("argument_" + fn.argnames.length),
+                        scope: fn
+                    });
+                    fn.argnames.push(argname);
+                    fn.enclosed.push(fn.def_variable(argname));
+                }
+            }
+            if (argname && find_if(function(node) {
+                return node.name === argname.name;
+            }, fn.argnames) === argname) {
+                var sym = make_node(AST_SymbolRef, self, argname);
+                sym.reference({});
+                delete argname.__unused;
+                return sym;
+            }
+        }
         if (is_lhs(self, compressor.parent())) return self;
         if (key !== prop) {
             var sub = self.flatten_object(property, compressor);
@@ -6251,31 +6293,6 @@ merge(Compressor.prototype, {
                 });
             }
         }
-        var fn;
-        if (compressor.option("arguments")
-            && expr instanceof AST_SymbolRef
-            && expr.name == "arguments"
-            && expr.definition().orig.length == 1
-            && (fn = expr.scope) instanceof AST_Lambda
-            && prop instanceof AST_Number) {
-            var index = prop.getValue();
-            var argname = fn.argnames[index];
-            if (!argname && !compressor.option("keep_fargs")) {
-                while (index >= fn.argnames.length) {
-                    argname = make_node(AST_SymbolFunarg, fn, {
-                        name: fn.make_var_name("argument_" + fn.argnames.length),
-                        scope: fn
-                    });
-                    fn.argnames.push(argname);
-                    fn.enclosed.push(fn.def_variable(argname));
-                }
-            }
-            if (argname) {
-                var sym = make_node(AST_SymbolRef, self, argname);
-                sym.reference({});
-                return sym;
-            }
-        }
         var ev = self.evaluate(compressor);
         if (ev !== self) {
             ev = make_node_from_constant(ev, self).optimize(compressor);
diff --git a/lib/utils.js b/lib/utils.js
index ca4b2d4b..7a51fb80 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -52,9 +52,7 @@ function member(name, array) {
 }
 
 function find_if(func, array) {
-    for (var i = 0, n = array.length; i < n; ++i) {
-        if (func(array[i])) return array[i];
-    }
+    for (var i = array.length; --i >= 0;) if (func(array[i])) return array[i];
 }
 
 function repeat_string(str, i) {