Don't define prototype on arrow functions

Summary: Functions defined with the arrow syntax should not have a `prototype` defined on them, as they cannot be used as constructors.

Reviewed By: avp

Differential Revision: D42178843

fbshipit-source-id: 7031f9e43b92cd6aa3698866025a02e775202f39

lib/VM/Callable.cpp

@@ -82,7 +82,12 @@ void Callable::defineLazyProperties(Handle<Callable> fn, Runtime &runtime) {
 
     // According to ES12 26.7.4, AsyncFunction instances do not have a
     // 'prototype' property, hence we need to set an null handle here.
-    auto prototypeObjectHandle = vmisa<JSAsyncFunction>(*jsFun)
+    // Functions that cannot be used with `new` should also not define a
+    // 'prototype' property, such as arrow functions per 15.3.4, except for
+    // generator functions.
+    auto prototypeObjectHandle =
+        codeBlock->getHeaderFlags().isCallProhibited(/* construct */ true) &&
+            !vmisa<JSGeneratorFunction>(*jsFun)
         ? Runtime::makeNullHandle<JSObject>()
         : runtime.makeHandle(JSObject::create(runtime, prototypeParent));
 

test/hermes/function-props.js

@@ -21,3 +21,10 @@ print(isNaN.name);
 //CHECK: isNaN
 print(Function.name);
 //CHECK: Function
+
+var a_func = function(){};
+print(a_func.hasOwnProperty("prototype"));
+//CHECK: true
+var a_arrow = () => {};
+print(a_arrow.hasOwnProperty("prototype"));
+//CHECK: false