enhance `dead_code` (#5014)

3 files changed, 231 insertions, 27 deletions

diff --git a/lib/compress.js b/lib/compress.js
index 22434f62..5a7461a8 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1714,6 +1714,15 @@ merge(Compressor.prototype, {
         return stat instanceof AST_LambdaDefinition;
     }
 
+    function is_last_statement(body, stat) {
+        var index = body.lastIndexOf(stat);
+        if (index < 0) return false;
+        while (++index < body.length) {
+            if (!is_declaration(body[index], true)) return false;
+        }
+        return true;
+    }
+
     function tighten_body(statements, compressor) {
         var in_loop, in_try, scope;
         find_loop_scope_try();
@@ -3168,15 +3177,6 @@ merge(Compressor.prototype, {
                 return !value || value instanceof AST_UnaryPrefix && value.operator == "void";
             }
 
-            function is_last_statement(body, stat) {
-                var index = body.lastIndexOf(stat);
-                if (index < 0) return false;
-                while (++index < body.length) {
-                    if (!is_declaration(body[index], true)) return false;
-                }
-                return true;
-            }
-
             function last_of(predicate) {
                 var block = self, stat, level = 0;
                 do {
@@ -11140,28 +11140,36 @@ merge(Compressor.prototype, {
                             if (node instanceof AST_PropAccess) return true;
                             if (!found && node instanceof AST_SymbolRef && node.definition() === def) {
                                 if (in_try(level, parent)) return true;
-                                def.fixed = false;
                                 found = true;
                             }
                         })) break;
-                        if (found) return strip_assignment();
-                    } else if (parent instanceof AST_Exit) {
+                        if (!found) continue;
+                        return strip_assignment(def);
+                    }
+                    if (parent instanceof AST_Exit) {
                         if (!local) break;
                         if (in_try(level, parent)) break;
                         if (is_reachable(scope, [ def ])) break;
-                        def.fixed = false;
-                        return strip_assignment();
-                    } else if (parent instanceof AST_VarDef) {
+                        return strip_assignment(def);
+                    }
+                    if (parent instanceof AST_SimpleStatement) {
+                        if (!local) break;
+                        if (is_reachable(scope, [ def ])) break;
+                        var stat;
+                        do {
+                            stat = parent;
+                            parent = compressor.parent(level++);
+                            if (parent === scope && is_last_statement(parent.body, stat)) return strip_assignment(def);
+                        } while (is_tail_block(stat, parent));
+                        break;
+                    }
+                    if (parent instanceof AST_VarDef) {
                         if (!(parent.name instanceof AST_SymbolDeclaration)) continue;
                         if (parent.name.definition() !== def) continue;
                         if (in_try(level, parent)) break;
-                        def.fixed = false;
-                        return strip_assignment();
+                        return strip_assignment(def);
                     }
-                } while (parent instanceof AST_Binary && parent.right === node
-                    || parent instanceof AST_Conditional && parent.condition !== node
-                    || parent instanceof AST_Sequence && parent.tail_node() === node
-                    || parent instanceof AST_UnaryPrefix);
+                } while (is_tail(node, parent));
             }
         }
         if (compressor.option("sequences")) {
@@ -11206,26 +11214,55 @@ merge(Compressor.prototype, {
         }
         return try_evaluate(compressor, self);
 
+        function is_tail(node, parent) {
+            if (parent instanceof AST_Binary) {
+                return parent.right === node || parent.right.is_constant_expression(scope);
+            }
+            if (parent instanceof AST_Conditional) {
+                return parent.condition !== node
+                    || parent.consequent.is_constant_expression(scope)
+                        && parent.alternative.is_constant_expression(scope);
+            }
+            if (parent instanceof AST_Sequence) {
+                var exprs = parent.expressions;
+                var stop = exprs.indexOf(node);
+                if (stop < 0) return false;
+                for (var i = exprs.length; --i > stop;) {
+                    if (!exprs[i].is_constant_expression(scope)) return false;
+                }
+                return true;
+            }
+            if (parent instanceof AST_UnaryPrefix) return true;
+        }
+
+        function is_tail_block(stat, parent) {
+            if (parent instanceof AST_BlockStatement) return is_last_statement(parent.body, stat);
+            if (parent instanceof AST_Catch) return is_last_statement(parent.body, stat);
+            if (parent instanceof AST_Finally) return is_last_statement(parent.body, stat);
+            if (parent instanceof AST_If) return parent.body === stat || parent.alternative === stat;
+            if (parent instanceof AST_Try) return parent.bfinally ? parent.bfinally === stat : parent.bcatch === stat;
+        }
+
         function in_try(level, node) {
             var right = self.right;
             self.right = make_node(AST_Null, right);
             var may_throw = node.may_throw(compressor);
             self.right = right;
-            var parent;
-            while (parent = compressor.parent(level++)) {
+            for (var parent; parent = compressor.parent(level++); node = parent) {
                 if (parent === scope) return false;
                 if (parent instanceof AST_Try) {
-                    if (parent.bfinally) return true;
-                    if (may_throw && parent.bcatch) return true;
+                    if (parent.bfinally && parent.bfinally !== node) return true;
+                    if (may_throw && parent.bcatch && parent.bcatch !== node) return true;
                 }
             }
         }
 
-        function strip_assignment() {
+        function strip_assignment(def) {
+            if (def) def.fixed = false;
             return (self.operator != "=" ? make_node(AST_Binary, self, {
                 operator: self.operator.slice(0, -1),
                 left: self.left,
-                right: self.right
+                right: self.right,
             }) : maintain_this_binding(compressor, compressor.parent(), self, self.right)).optimize(compressor);
         }
     });
diff --git a/test/compress/dead-code.js b/test/compress/dead-code.js
index 9101869d..4493deb0 100644
--- a/test/compress/dead-code.js
+++ b/test/compress/dead-code.js
@@ -939,6 +939,170 @@ catch_return_assign: {
     expect_stdout: "PASS"
 }
 
+catch_return_assign_may_throw: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function f() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                return e = console.log("PASS");
+            }
+        }
+        f();
+    }
+    expect: {
+        function f() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                return console.log("PASS");
+            }
+        }
+        f();
+    }
+    expect_stdout: "PASS"
+}
+
+finally_return_assign: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        console.log(function(a) {
+            try {
+                throw "FAIL";
+            } finally {
+                return a = "PASS";
+            }
+        }());
+    }
+    expect: {
+        console.log(function(a) {
+            try {
+                throw "FAIL";
+            } finally {
+                return "PASS";
+            }
+        }());
+    }
+    expect_stdout: "PASS"
+}
+
+last_assign_statement: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function f(a) {
+            a = a("PASS");
+        }
+        f(console.log);
+    }
+    expect: {
+        function f(a) {
+            a("PASS");
+        }
+        f(console.log);
+    }
+    expect_stdout: "PASS"
+}
+
+last_assign_if_else: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function f(a) {
+            if (a)
+                a = console.log("foo");
+            else {
+                console.log("bar");
+                a = console.log("baz");
+            }
+        }
+        f(42);
+        f(null);
+    }
+    expect: {
+        function f(a) {
+            if (a)
+                console.log("foo");
+            else {
+                console.log("bar");
+                console.log("baz");
+            }
+        }
+        f(42);
+        f(null);
+    }
+    expect_stdout: [
+        "foo",
+        "bar",
+        "baz",
+    ]
+}
+
+last_assign_catch: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function f() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                e = console.log("PASS");
+            }
+        }
+        f();
+    }
+    expect: {
+        function f() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                console.log("PASS");
+            }
+        }
+        f();
+    }
+    expect_stdout: "PASS"
+}
+
+last_assign_finally: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        function f(a) {
+            try {
+                throw a.log;
+            } catch (e) {
+                a = e;
+            } finally {
+                a = a("PASS");
+            }
+        }
+        f(console);
+    }
+    expect: {
+        function f(a) {
+            try {
+                throw a.log;
+            } catch (e) {
+                a = e;
+            } finally {
+                a("PASS");
+            }
+        }
+        f(console);
+    }
+    expect_stdout: "PASS"
+}
+
 issue_3578: {
     options = {
         dead_code: true,
diff --git a/test/release/rollup-ts.sh b/test/release/rollup-ts.sh
index 503aeadf..4763911d 100755
--- a/test/release/rollup-ts.sh
+++ b/test/release/rollup-ts.sh
@@ -43,6 +43,9 @@ rm -rf tmp/rollup \
 -    "postpublish": "pinst --enable",
 -    "prepare": "npm run build",
 -    "prepublishOnly": "pinst --disable && npm ci && npm run lint:nofix && npm run security && npm run build:bootstrap && npm run test:all",
+@@ -93 +89 @@
+-    "is-reference": "lukastaegert/is-reference#update-class-features",
++    "is-reference": "3.0.0",
 --- a/test/cli/index.js
 +++ b/test/cli/index.js
 @@ -13,0 +14,3 @@ sander.rimrafSync(__dirname, 'node_modules');