Add warning when using non-component element types

This should make debugging easier for people who missed callsites when fixing up their JSX/non-JSX usage.

src/core/ReactCompositeComponent.js

@@ -133,6 +133,20 @@ var ReactCompositeComponentMixin = {
 
     // Initialize the public class
     var inst = new Component(publicProps, publicContext);
+
+    if (__DEV__) {
+      // This will throw later in _renderValidatedComponent, but add an early
+      // warning now to help debugging
+      warning(
+        inst.render != null,
+        '%s(...): No `render` method found on the returned component ' +
+        'instance: you may have forgotten to define `render` in your ' +
+        'component or you may have accidentally tried to render an element ' +
+        'whose type is a function that isn\'t a React component.',
+        Component.displayName || Component.name || 'Component'
+      );
+    }
+
     // These should be set up in the constructor, but as a convenience for
     // simpler class abstractions, we set them up after the fact.
     inst.props = publicProps;

src/core/__tests__/ReactCompositeComponent-test.js

@@ -954,4 +954,17 @@ describe('ReactCompositeComponent', function() {
     expect(a).toBe(b);
   });
 
+  it('should warn when using non-React functions in JSX', function() {
+    function NotAComponent() {
+      return [<div />, <div />];
+    }
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(<div><NotAComponent /></div>);
+    }).toThrow();  // has no method 'render'
+    expect(console.warn.calls.length).toBe(1);
+    expect(console.warn.calls[0].args[0]).toContain(
+      'NotAComponent(...): No `render` method found'
+    );
+  });
+
 });