fix corner case in comparisons (#5486)

fixes #5485

README.md

@@ -1407,3 +1407,17 @@ To allow for better optimizations, the compiler makes various assumptions:
   // Actual:   SyntaxError: Unexpected reserved word
   ```
   UglifyJS may modify the input which in turn may suppress those errors.
+- Later versions of Chrome and Node.js will give incorrect results with the
+  following:
+  ```javascript
+  try {
+      f();
+      function f() {
+          throw 42;
+      }
+  } catch (e) {}
+  console.log(typeof f);
+  // Expected: "function"
+  // Actual:   "undefined"
+  ```
+  UglifyJS may modify the input which in turn may suppress those errors.

lib/compress.js

@@ -1559,7 +1559,10 @@ Compressor.prototype.compress = function(node) {
 
     AST_SymbolRef.DEFMETHOD("is_immutable", function() {
         var def = this.redef || this.definition();
-        return (this.in_arg || def.orig.length == 1) && def.orig[0] instanceof AST_SymbolLambda;
+        if (!(def.orig[0] instanceof AST_SymbolLambda)) return false;
+        if (def.orig.length == 1) return true;
+        if (!this.in_arg) return false;
+        return !(def.orig[1] instanceof AST_SymbolFunarg);
     });
 
     AST_Node.DEFMETHOD("convert_symbol", noop);

test/compress/default-values.js

@@ -2448,3 +2448,17 @@ issue_5465: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5485: {
+    options = {
+        comparisons: true,
+    }
+    input: {
+        (function f(f, a = console.log(void 0 === f ? "PASS" : "FAIL")) {})();
+    }
+    expect: {
+        (function f(f, a = console.log(void 0 === f ? "PASS" : "FAIL")) {})();
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}

test/compress/destructured.js

@@ -3626,3 +3626,23 @@ issue_5454: {
     expect_stdout: "PASS"
     node_version: ">=6"
 }
+
+issue_5485: {
+    options = {
+        comparisons: true,
+    }
+    input: {
+        (function f({
+            p: f,
+            [console.log(void 0 === f ? "PASS" : "FAIL")]: a,
+        }) {})(42);
+    }
+    expect: {
+        (function f({
+            p: f,
+            [console.log(void 0 === f ? "PASS" : "FAIL")]: a,
+        }) {})(42);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}