From f1eb03f2c0f860a963b2f61c5f8565d8703a18cb Mon Sep 17 00:00:00 2001
From: "Alex Lam S.L" <alexlamsl@gmail.com>
Date: Wed, 30 Oct 2019 06:34:54 +0800
Subject: [PATCH] enhance `dead_code` (#3551)

---
 lib/compress.js              | 51 +++++++++++++++++++++++-------------
 test/compress/dead-code.js   | 29 ++++++++++++++++++++
 test/compress/drop-unused.js | 34 ++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 18 deletions(-)

diff --git a/lib/compress.js b/lib/compress.js
index ad47621a..33943ff0 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -3441,6 +3441,7 @@ merge(Compressor.prototype, {
                     return true;
                 }
                 if (node instanceof AST_Scope) {
+                    if (node === self) return;
                     scopes.push(node);
                     descend();
                     scopes.pop();
@@ -3451,6 +3452,7 @@ merge(Compressor.prototype, {
                         result = false;
                         return true;
                     }
+                    if (self.variables.has(node.name)) return true;
                     var def = node.definition();
                     if (member(def.scope, scopes)) return true;
                     if (scope) {
@@ -6429,24 +6431,37 @@ merge(Compressor.prototype, {
     var ASSIGN_OPS = makePredicate("+ - / * % >> << >>> | ^ &");
     var ASSIGN_OPS_COMMUTATIVE = makePredicate("* | ^ &");
     OPT(AST_Assign, function(self, compressor) {
-        var def;
-        if (compressor.option("dead_code")
-            && self.left instanceof AST_SymbolRef
-            && (def = self.left.definition()).scope === compressor.find_parent(AST_Lambda)) {
-            if (self.left.is_immutable()) return strip_assignment();
-            var level = 0, node, parent = self;
-            do {
-                node = parent;
-                parent = compressor.parent(level++);
-                if (parent instanceof AST_Exit) {
-                    if (in_try(level, parent)) break;
-                    if (is_reachable(def.scope, [ def ])) break;
-                    def.fixed = false;
-                    return strip_assignment();
-                }
-            } while (parent instanceof AST_Binary && parent.right === node
-                || parent instanceof AST_Sequence && parent.tail_node() === node
-                || parent instanceof AST_UnaryPrefix);
+        if (compressor.option("dead_code")) {
+            if (self.left instanceof AST_PropAccess) {
+                var exp = self.left.expression;
+                if (exp instanceof AST_Lambda
+                    || !compressor.has_directive("use strict")
+                        && exp instanceof AST_Constant
+                        && !exp.may_throw_on_access(compressor)) {
+                    return self.left instanceof AST_Dot ? self.right : make_sequence(self, [
+                        self.left.property,
+                        self.right
+                    ]).optimize(compressor);
+                }
+            } else if (self.left instanceof AST_SymbolRef) {
+                var def = self.left.definition();
+                if (def.scope === compressor.find_parent(AST_Lambda)) {
+                    if (self.left.is_immutable()) return strip_assignment();
+                    var level = 0, node, parent = self;
+                    do {
+                        node = parent;
+                        parent = compressor.parent(level++);
+                        if (parent instanceof AST_Exit) {
+                            if (in_try(level, parent)) break;
+                            if (is_reachable(def.scope, [ def ])) break;
+                            def.fixed = false;
+                            return strip_assignment();
+                        }
+                    } while (parent instanceof AST_Binary && parent.right === node
+                        || parent instanceof AST_Sequence && parent.tail_node() === node
+                        || parent instanceof AST_UnaryPrefix);
+                }
+            }
         }
         self = self.lift_sequences(compressor);
         if (!compressor.option("assignments")) return self;
diff --git a/test/compress/dead-code.js b/test/compress/dead-code.js
index 190a3243..501d55ec 100644
--- a/test/compress/dead-code.js
+++ b/test/compress/dead-code.js
@@ -1013,3 +1013,32 @@ issue_3406: {
     }
     expect_stdout: "true"
 }
+
+function_assign: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        console.log(function() {
+            var a = "PASS";
+            function h(c) {
+                return c;
+            }
+            h.p = function(b) {
+                return b;
+            }.p = a;
+            return h;
+        }().p);
+    }
+    expect: {
+        console.log(function() {
+            var a = "PASS";
+            function h(c) {
+                return c;
+            }
+            h.p = a;
+            return h;
+        }().p);
+    }
+    expect_stdout: "PASS"
+}
diff --git a/test/compress/drop-unused.js b/test/compress/drop-unused.js
index f77072b0..7cb2e899 100644
--- a/test/compress/drop-unused.js
+++ b/test/compress/drop-unused.js
@@ -2187,3 +2187,37 @@ issue_3515_3: {
     }
     expect_stdout: "PASS"
 }
+
+function_assign: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            var a = "PASS";
+            function g(b) {
+                return b;
+            }
+            g.p = a;
+            function h(c) {
+                return c;
+            }
+            h.p = a;
+            return h;
+        }().p);
+    }
+    expect: {
+        console.log(function() {
+            var a = "PASS";
+            function h(c) {
+                return c;
+            }
+            h.p = a;
+            return h;
+        }().p);
+    }
+    expect_stdout: "PASS"
+}
-- 
2.34.1