Constructor error message (#11395)

* Constructor test and fix complete

* Linters and prettier run

* Remove unnecessary checks

* Update error message

* Updat unit test

* prettier

* Tweak the check to be more specific

* Move tests to ReactCompositeComponent-test

* add error call count and remove line

Author: dylanapplegate

packages/react-dom/src/__tests__/ReactCompositeComponent-test.js

@@ -1533,4 +1533,43 @@ describe('ReactCompositeComponent', () => {
     ReactTestUtils.renderIntoDocument(<Component />);
     expect(mockArgs.length).toEqual(0);
   });
+
+  it('should return a meaningful warning when constructor is returned', () => {
+    spyOn(console, 'error');
+    class RenderTextInvalidConstructor extends React.Component {
+      constructor(props) {
+        super(props);
+        return {something: false};
+      }
+
+      render() {
+        return <div />;
+      }
+    }
+
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(<RenderTextInvalidConstructor />);
+    }).toThrow();
+
+    expectDev(console.error.calls.count()).toBe(1);
+    expectDev(console.error.calls.mostRecent().args[0]).toBe(
+      'Warning: RenderTextInvalidConstructor(...): No `render` method found on the returned component instance: ' +
+        'did you accidentally return an object from the constructor?',
+    );
+  });
+
+  it('should return error if render is not defined', () => {
+    spyOn(console, 'error');
+    class RenderTestUndefinedRender extends React.Component {}
+
+    expect(function() {
+      ReactTestUtils.renderIntoDocument(<RenderTestUndefinedRender />);
+    }).toThrow();
+
+    expectDev(console.error.calls.count()).toBe(1);
+    expectDev(console.error.calls.mostRecent().args[0]).toBe(
+      'Warning: RenderTestUndefinedRender(...): No `render` method found on the returned ' +
+        'component instance: you may have forgotten to define `render`.',
+    );
+  });
 });

packages/react-reconciler/src/ReactFiberClassComponent.js

@@ -202,12 +202,25 @@ module.exports = function(
     if (__DEV__) {
       const name = getComponentName(workInProgress);
       const renderPresent = instance.render;
-      warning(
-        renderPresent,
-        '%s(...): No `render` method found on the returned component ' +
-          'instance: you may have forgotten to define `render`.',
-        name,
-      );
+
+      if (!renderPresent) {
+        if (type.prototype && typeof type.prototype.render === 'function') {
+          warning(
+            false,
+            '%s(...): No `render` method found on the returned component ' +
+              'instance: did you accidentally return an object from the constructor?',
+            name,
+          );
+        } else {
+          warning(
+            false,
+            '%s(...): No `render` method found on the returned component ' +
+              'instance: you may have forgotten to define `render`.',
+            name,
+          );
+        }
+      }
+
       const noGetInitialStateOnES6 =
         !instance.getInitialState ||
         instance.getInitialState.isReactClassApproved ||