Support native arrow functions as stateless components

src/renderers/shared/reconciler/ReactCompositeComponent.js

@@ -42,7 +42,7 @@ function StatelessComponent(Component) {
 }
 StatelessComponent.prototype.render = function() {
   var Component = ReactInstanceMap.get(this)._currentElement.type;
-  return new Component(this.props, this.context, this.updater);
+  return Component(this.props, this.context, this.updater);
 };
 
 /**
@@ -136,18 +136,26 @@ var ReactCompositeComponentMixin = {
     var inst;
     var renderedElement;
 
-    if (__DEV__) {
-      ReactCurrentOwner.current = this;
-      try {
+    // This is a way to detect if Component is a stateless arrow function
+    // component, which is not newable. It might not be 100% reliable but is
+    // something we can do until we start detecting that Component extends
+    // React.Component. We already assume that typeof Component === 'function'.
+    var canInstantiate = 'prototype' in Component;
+
+    if (canInstantiate) {
+      if (__DEV__) {
+        ReactCurrentOwner.current = this;
+        try {
+          inst = new Component(publicProps, publicContext, ReactUpdateQueue);
+        } finally {
+          ReactCurrentOwner.current = null;
+        }
+      } else {
         inst = new Component(publicProps, publicContext, ReactUpdateQueue);
-      } finally {
-        ReactCurrentOwner.current = null;
       }
-    } else {
-      inst = new Component(publicProps, publicContext, ReactUpdateQueue);
     }
 
-    if (inst === null || inst === false || ReactElement.isValidElement(inst)) {
+    if (!canInstantiate || inst === null || inst === false || ReactElement.isValidElement(inst)) {
       renderedElement = inst;
       inst = new StatelessComponent(Component);
     }
@@ -168,7 +176,9 @@ var ReactCompositeComponentMixin = {
         // We support ES6 inheriting from React.Component, the module pattern,
         // and stateless components, but not ES6 classes that don't extend
         warning(
-          Component.isReactClass || !(inst instanceof Component),
+          Component.isReactClass ||
+            !canInstantiate ||
+            !(inst instanceof Component),
           '%s(...): React component classes must extend React.Component.',
           Component.displayName || Component.name || 'Component'
         );

src/renderers/shared/reconciler/__tests__/ReactStatelessComponent-test.js

@@ -183,4 +183,19 @@ describe('ReactStatelessComponent', function() {
     ReactDOM.render(<Parent />, el);
     expect(el.textContent).toBe('en');
   });
+
+  it('should work with arrow functions', function() {
+    // TODO: actually use arrow functions, probably need node v4 and maybe
+    // a separate file that we blacklist from the arrow function transform.
+    // We can't actually test this without native arrow functions since the
+    // issues (non-newable) don't apply to any other functions.
+    var Child = function() {
+      return <div />;
+    };
+    // Will create a new bound function without a prototype, much like a native
+    // arrow function.
+    Child = Child.bind(this);
+
+    expect(() => ReactTestUtils.renderIntoDocument(<Child />)).not.toThrow();
+  });
 });