Move ref type check to receiver

The runtime contains a type check to determine if a user-provided ref
is a valid type — a function or object (or a string, when
`disableStringRefs` is off). This currently happens during child
reconciliation. This changes it to happen only when the ref is passed
to the component that the ref is being attached to.

This is a continuation of the "ref as prop" change — until you actually
pass a ref to a HostComponent, class, etc, ref is a normal prop that has
no special behavior.

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

@@ -46,7 +46,10 @@ describe('ReactComponent', () => {
       act(() => {
         root.render(<div ref="badDiv" />);
       }),
-    ).rejects.toThrow();
+    ).rejects.toThrow(
+      'Element ref was specified as a string (badDiv) but no owner ' +
+        'was set',
+    );
   });
 
   it('should throw (in dev) when children are mutated during render', async () => {

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

@@ -401,14 +401,14 @@ describe('ref swapping', () => {
         root.render(<div ref={10} />);
       });
     }).rejects.toThrow(
-      'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
+      'Element ref was specified as a string (10) but no owner was set.',
     );
     await expect(async () => {
       await act(() => {
         root.render(<div ref={true} />);
       });
     }).rejects.toThrow(
-      'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
+      'Element ref was specified as a string (true) but no owner was set.',
     );
     await expect(async () => {
       await act(() => {

packages/react-reconciler/src/ReactChildFiber.js

@@ -157,17 +157,17 @@ function convertStringRefToCallbackRef(
   returnFiber: Fiber,
   current: Fiber | null,
   element: ReactElement,
-  mixedRef: any,
+  mixedRef: string | number | boolean,
 ): CoercedStringRef {
+  if (__DEV__) {
+    checkPropStringCoercion(mixedRef, 'ref');
+  }
+  const stringRef = '' + (mixedRef: any);
+
   const owner: ?Fiber = (element._owner: any);
   if (!owner) {
-    if (typeof mixedRef !== 'string') {
-      throw new Error(
-        'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
-      );
-    }
     throw new Error(
-      `Element ref was specified as a string (${mixedRef}) but no owner was set. This could happen for one of` +
+      `Element ref was specified as a string (${stringRef}) but no owner was set. This could happen for one of` +
         ' the following reasons:\n' +
         '1. You may be adding a ref to a function component\n' +
         "2. You may be adding a ref to a component that was not created inside a component's render method\n" +
@@ -184,13 +184,6 @@ function convertStringRefToCallbackRef(
     );
   }
 
-  // At this point, we know the ref isn't an object or function but it could
-  // be a number. Coerce it to a string.
-  if (__DEV__) {
-    checkPropStringCoercion(mixedRef, 'ref');
-  }
-  const stringRef = '' + mixedRef;
-
   if (__DEV__) {
     if (
       // Will already warn with "Function components cannot be given refs"
@@ -267,12 +260,10 @@ function coerceRef(
   let coercedRef;
   if (
     !disableStringRefs &&
-    mixedRef !== null &&
-    typeof mixedRef !== 'function' &&
-    typeof mixedRef !== 'object'
+    (typeof mixedRef === 'string' ||
+      typeof mixedRef === 'number' ||
+      typeof mixedRef === 'boolean')
   ) {
-    // Assume this is a string ref. If it's not, then this will throw an error
-    // to the user.
     coercedRef = convertStringRefToCallbackRef(
       returnFiber,
       current,

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -1026,16 +1026,24 @@ function updateProfiler(
 }
 
 function markRef(current: Fiber | null, workInProgress: Fiber) {
-  // TODO: This is also where we should check the type of the ref and error if
-  // an invalid one is passed, instead of during child reconcilation.
+  // TODO: Check props.ref instead of fiber.ref when enableRefAsProp is on.
   const ref = workInProgress.ref;
-  if (
-    (current === null && ref !== null) ||
-    (current !== null && current.ref !== ref)
-  ) {
-    // Schedule a Ref effect
-    workInProgress.flags |= Ref;
-    workInProgress.flags |= RefStatic;
+  if (ref === null) {
+    if (current !== null && current.ref !== null) {
+      // Schedule a Ref effect
+      workInProgress.flags |= Ref | RefStatic;
+    }
+  } else {
+    if (typeof ref !== 'function' && typeof ref !== 'object') {
+      // TODO: Remove "string" from error message.
+      throw new Error(
+        'Expected ref to be a function, a string, an object returned by React.createRef(), or null.',
+      );
+    }
+    if (current === null || current.ref !== ref) {
+      // Schedule a Ref effect
+      workInProgress.flags |= Ref | RefStatic;
+    }
   }
 }
 

packages/react-reconciler/src/__tests__/ReactFiberRefs-test.js

@@ -115,9 +115,13 @@ describe('ReactFiberRefs', () => {
   });
 
   // @gate disableStringRefs
-  test('log an error in dev if a string ref is passed to a ref-receiving component', async () => {
+  test('throw if a string ref is passed to a ref-receiving component', async () => {
     let refProp;
     function Child({ref}) {
+      // This component renders successfully because the ref type check does not
+      // occur until you pass it to a component that accepts refs.
+      //
+      // So the div will throw, but not Child.
       refProp = ref;
       return <div ref={ref} />;
     }
@@ -129,9 +133,9 @@ describe('ReactFiberRefs', () => {
     }
 
     const root = ReactNoop.createRoot();
-    await expect(async () => {
-      await expect(act(() => root.render(<Owner />))).rejects.toThrow();
-    }).toErrorDev('String refs are no longer supported');
+    await expect(act(() => root.render(<Owner />))).rejects.toThrow(
+      'Expected ref to be a function',
+    );
     expect(refProp).toBe('child');
   });
 });