enhance dead_code (#3575)

lib/compress.js

@@ -6543,14 +6543,20 @@ merge(Compressor.prototype, {
                     }
                 }
             } 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 def = self.left.definition();
+                var local = def.scope.resolve() === compressor.find_parent(AST_Lambda);
                 var level = 0, node, parent = self;
                 do {
                     node = parent;
                     parent = compressor.parent(level++);
-                        if (parent instanceof AST_Exit) {
+                    if (parent instanceof AST_Assign) {
+                        if (!(parent.left instanceof AST_SymbolRef)) continue;
+                        if (parent.left.definition() !== def) continue;
+                        def.fixed = false;
+                        return strip_assignment();
+                    } else if (parent instanceof AST_Exit) {
+                        if (!local) break;
                         if (in_try(level, parent)) break;
                         if (is_reachable(def.scope, [ def ])) break;
                         def.fixed = false;
@@ -6561,7 +6567,6 @@ merge(Compressor.prototype, {
                     || parent instanceof AST_UnaryPrefix);
             }
         }
-        }
         self = self.lift_sequences(compressor);
         if (!compressor.option("assignments")) return self;
         if (self.operator == "=" && self.left instanceof AST_SymbolRef && self.right instanceof AST_Binary) {
@@ -6599,7 +6604,7 @@ merge(Compressor.prototype, {
             self.right = make_node(AST_Null, right);
             var may_throw = node.may_throw(compressor);
             self.right = right;
-            var scope = self.left.definition().scope;
+            var scope = self.left.definition().scope.resolve();
             var parent;
             while ((parent = compressor.parent(level++)) !== scope) {
                 if (parent instanceof AST_Try) {

test/compress/dead-code.js

@@ -1064,3 +1064,41 @@ issue_3552: {
     }
     expect_stdout: "PASS"
 }
+
+unreachable_assign: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        console.log(A = "P" + (A = "A" + (B = "S" + (A = B = "S"))), A, B);
+    }
+    expect: {
+        console.log(A = "P" + "A" + (B = "S" + "S"), A, B);
+    }
+    expect_stdout: "PASS PASS SS"
+}
+
+catch_return_assign: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        console.log(function() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                return e = "PASS";
+            }
+        }());
+    }
+    expect: {
+        console.log(function() {
+            try {
+                throw "FAIL";
+            } catch (e) {
+                return "PASS";
+            }
+        }());
+    }
+    expect_stdout: "PASS"
+}