fix corner case in evaluate (#4501)

fixes #4500

lib/compress.js

@@ -3894,7 +3894,13 @@ merge(Compressor.prototype, {
             if (node instanceof AST_Unary && UNARY_POSTFIX[node.operator]) modified(node.expression);
         });
         function modified(node) {
-            if (node instanceof AST_PropAccess) {
+            if (node instanceof AST_DestructuredArray) {
+                node.elements.forEach(modified);
+            } else if (node instanceof AST_DestructuredObject) {
+                node.properties.forEach(function(prop) {
+                    modified(prop.value);
+                });
+            } else if (node instanceof AST_PropAccess) {
                 modified(node.expression);
             } else if (node instanceof AST_SymbolRef) {
                 node.definition().references.forEach(function(ref) {
@@ -3928,7 +3934,6 @@ merge(Compressor.prototype, {
             if (!HOP(lhs, "_eval") && lhs instanceof AST_SymbolRef && lhs.fixed && lhs.definition().fixed) {
                 node = lhs;
             } else if (op == "=") {
-                lhs.walk(scan_modified);
                 node = this.right;
             } else {
                 node = make_node(AST_Binary, this, {
@@ -3937,6 +3942,7 @@ merge(Compressor.prototype, {
                     right: this.right,
                 });
             }
+            lhs.walk(scan_modified);
             var value = node._eval(compressor, ignore_side_effects, cached, depth);
             if (typeof value == "object") return this;
             modified(lhs);
@@ -4011,6 +4017,7 @@ merge(Compressor.prototype, {
             }
             var def = e instanceof AST_SymbolRef && e.definition();
             if (!non_converting_unary[op] && !(def && def.fixed)) depth++;
+            e.walk(scan_modified);
             var v = e._eval(compressor, ignore_side_effects, cached, depth);
             if (v === e) {
                 if (ignore_side_effects && op == "void") return;
@@ -4054,6 +4061,7 @@ merge(Compressor.prototype, {
                 }
             }
             if (!(e instanceof AST_SymbolRef && e.definition().fixed)) depth++;
+            e.walk(scan_modified);
             var v = e._eval(compressor, ignore_side_effects, cached, depth);
             if (v === e) return this;
             modified(e);

test/compress/destructured.js

@@ -2302,3 +2302,27 @@ issue_4485_3: {
     expect_stdout: true
     node_version: ">=6"
 }
+
+issue_4500: {
+    options = {
+        evaluate: true,
+        keep_fnames: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = function f(b) {
+            return [ b ] = [], b;
+        }("FAIL");
+        console.log(a || "PASS");
+    }
+    expect: {
+        var a = function f(b) {
+            return [ b ] = [], b;
+        }("FAIL");
+        console.log(a || "PASS");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}