fix corner case in keep_fargs (#5477)

fixes #5476

lib/scope.js

@@ -64,19 +64,20 @@ SymbolDef.prototype = {
         this.references.forEach(fn);
     },
     mangle: function(options) {
-        var cache = options.cache && options.cache.props;
-        if (this.global && cache && cache.has(this.name)) {
+        if (this.mangled_name) return;
+        var cache = this.global && options.cache && options.cache.props;
+        if (cache && cache.has(this.name)) {
             this.mangled_name = cache.get(this.name);
-        } else if (!this.mangled_name && !this.unmangleable(options)) {
+        } else if (this.unmangleable(options)) {
+            names_in_use(this.scope, options).set(this.name, true);
+        } else {
             var def = this.redefined();
             if (def) {
                 this.mangled_name = def.mangled_name || def.name;
             } else {
                 this.mangled_name = next_mangled_name(this, options);
             }
-            if (this.global && cache) {
-                cache.set(this.name, this.mangled_name);
-            }
+            if (cache) cache.set(this.name, this.mangled_name);
         }
     },
     redefined: function() {

test/compress/const.js

@@ -1855,3 +1855,20 @@ issue_5338: {
     }
     expect_stdout: true
 }
+
+issue_5476: {
+    mangle = {
+        keep_fargs: true,
+    }
+    input: {
+        console.log(function(n) {
+            const a = 42;
+        }());
+    }
+    expect: {
+        console.log(function(n) {
+            const o = 42;
+        }());
+    }
+    expect_stdout: "undefined"
+}

test/compress/let.js

@@ -2020,3 +2020,23 @@ issue_5338: {
     expect_stdout: ReferenceError("a is not defined")
     node_version: ">=4"
 }
+
+issue_5476: {
+    mangle = {
+        keep_fargs: true,
+    }
+    input: {
+        "use strict";
+        console.log(function(n) {
+            let a;
+        }());
+    }
+    expect: {
+        "use strict";
+        console.log(function(n) {
+            let o;
+        }());
+    }
+    expect_stdout: "undefined"
+    node_version: ">=4"
+}

test/ufuzz/index.js

@@ -2329,7 +2329,7 @@ function is_error_tdz(ex) {
 }
 
 function is_error_spread(ex) {
-    return ex.name == "TypeError" && /Found non-callable @@iterator| is not iterable| not a function/.test(ex.message);
+    return ex.name == "TypeError" && /Found non-callable @@iterator| is not iterable| not a function|Symbol\(Symbol\.iterator\)/.test(ex.message);
 }
 
 function is_error_recursion(ex) {