Enhance dev warnings for forwardRef render function (facebook#13627) (f…

…acebook#13636)

* Enhance dev warnings for forwardRef render function

- For 0 parameters: Do not warn because it may be due to usage of the
  arguments object.

- For 1 parameter: Warn about missing the 'ref' parameter.

- For 2 parameters: This is the ideal. Do not warn.

- For more than 2 parameters: Warn about undefined parameters.

* Make test cases for forwardRef warnings more realistic

* Add period to warning sentence

packages/react/src/__tests__/forwardRef-test.js

@@ -162,22 +162,32 @@ describe('forwardRef', () => {
     );
   });
 
-  it('should warn if the render function provided does not use the forwarded ref parameter', () => {
-    function arityOfZero() {
-      return null;
-    }
+  it('should not warn if the render function provided does not use any parameter', () => {
+    const arityOfZero = () => <div ref={arguments[1]} />;
+    React.forwardRef(arityOfZero);
+  });
 
-    const arityOfOne = props => null;
+  it('should warn if the render function provided does not use the forwarded ref parameter', () => {
+    const arityOfOne = props => <div {...props} />;
 
-    expect(() => React.forwardRef(arityOfZero)).toWarnDev(
-      'forwardRef render functions accept two parameters: props and ref. ' +
+    expect(() => React.forwardRef(arityOfOne)).toWarnDev(
+      'forwardRef render functions accept exactly two parameters: props and ref. ' +
         'Did you forget to use the ref parameter?',
       {withoutStack: true},
     );
+  });
 
-    expect(() => React.forwardRef(arityOfOne)).toWarnDev(
-      'forwardRef render functions accept two parameters: props and ref. ' +
-        'Did you forget to use the ref parameter?',
+  it('should not warn if the render function provided use exactly two parameters', () => {
+    const arityOfTwo = (props, ref) => <div {...props} ref={ref} />;
+    React.forwardRef(arityOfTwo);
+  });
+
+  it('should warn if the render function provided expects to use more than two parameters', () => {
+    const arityOfThree = (props, ref, x) => <div {...props} ref={ref} x={x} />;
+
+    expect(() => React.forwardRef(arityOfThree)).toWarnDev(
+      'forwardRef render functions accept exactly two parameters: props and ref. ' +
+        'Any additional parameter will be undefined.',
       {withoutStack: true},
     );
   });

packages/react/src/forwardRef.js

@@ -21,9 +21,12 @@ export default function forwardRef<Props, ElementType: React$ElementType>(
       );
     } else {
       warningWithoutStack(
-        render.length === 2,
-        'forwardRef render functions accept two parameters: props and ref. ' +
-          'Did you forget to use the ref parameter?',
+        // Do not warn for 0 arguments because it could be due to usage of the 'arguments' object
+        render.length === 0 || render.length === 2,
+        'forwardRef render functions accept exactly two parameters: props and ref. %s',
+        render.length === 1
+          ? 'Did you forget to use the ref parameter?'
+          : 'Any additional parameter will be undefined.',
       );
     }