Don't parenthesize arrow functions in parameter lists

lib/output.js

@@ -597,6 +597,7 @@ function OutputStream(options) {
             || p instanceof AST_ObjectProperty   // { foo: (1, 2) }.foo ==> 2
             || p instanceof AST_Conditional      /* (false, true) ? (a = 10, b = 20) : (c = 30)
                                                   * ==> 20 (side effect, set a := 10 and b := 20) */
+            || p instanceof AST_Arrow            // x => (x, x)
         ;
     });
 
@@ -944,7 +945,7 @@ function OutputStream(options) {
         var parent = output.parent();
         var needs_parens = parent instanceof AST_Binary ||
             parent instanceof AST_Unary ||
-            parent instanceof AST_Call;
+            (parent instanceof AST_Call && self === parent.expression);
         if (needs_parens) { output.print("(") }
         if (self.argnames.length === 1 && self.argnames[0] instanceof AST_Symbol && !self.argnames[0].default) {
             self.argnames[0].print(output);

test/compress/harmony.js

@@ -391,3 +391,14 @@ regression_cannot_use_of: {
         foo();  /* Label statement missing? No prob. */
     }
 }
+
+fat_arrow_as_param: {
+    input: {
+        foo(x => x);
+        foo(x => x, y => y);
+
+        foo(x => (x, x));
+        foo(x => (x, x), y => (y, y));
+    }
+    expect_exact: "foo(x=>x);foo(x=>x,y=>y);foo(x=>(x,x));foo(x=>(x,x),y=>(y,y));"
+}