Don’t error when returning an empty Fragment (#12966)

* Don’t error when returning an empty Fragment

When a fragment is reconciled, we directly move onto it’s children.
Since an empty `<React.Fragment/>` will have children of `undefined`,
this would always throw.

To fix this, we bail out in those cases.

* Test the update path as well

* Reuse existing code path

* An even more explicit solution that also fixes Flow

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

@@ -171,6 +171,31 @@ describe('ReactDOMFiber', () => {
     expect(firstNode.tagName).toBe('DIV');
   });
 
+  it('renders an empty fragment', () => {
+    const Div = () => <div />;
+    const EmptyFragment = () => <React.Fragment />;
+    const NonEmptyFragment = () => (
+      <React.Fragment>
+        <Div />
+      </React.Fragment>
+    );
+
+    ReactDOM.render(<EmptyFragment />, container);
+    expect(container.firstChild).toBe(null);
+
+    ReactDOM.render(<NonEmptyFragment />, container);
+    expect(container.firstChild.tagName).toBe('DIV');
+
+    ReactDOM.render(<EmptyFragment />, container);
+    expect(container.firstChild).toBe(null);
+
+    ReactDOM.render(<Div />, container);
+    expect(container.firstChild.tagName).toBe('DIV');
+
+    ReactDOM.render(<EmptyFragment />, container);
+    expect(container.firstChild).toBe(null);
+  });
+
   let svgEls, htmlEls, mathEls;
   const expectSVG = {ref: el => svgEls.push(el)};
   const expectHTML = {ref: el => htmlEls.push(el)};

packages/react-reconciler/src/ReactChildFiber.js

@@ -1208,12 +1208,12 @@ function ChildReconciler(shouldTrackSideEffects) {
     // Handle top level unkeyed fragments as if they were arrays.
     // This leads to an ambiguity between <>{[...]}</> and <>...</>.
     // We treat the ambiguous cases above the same.
-    if (
+    const isUnkeyedTopLevelFragment =
       typeof newChild === 'object' &&
       newChild !== null &&
       newChild.type === REACT_FRAGMENT_TYPE &&
-      newChild.key === null
-    ) {
+      newChild.key === null;
+    if (isUnkeyedTopLevelFragment) {
       newChild = newChild.props.children;
     }
 
@@ -1281,7 +1281,7 @@ function ChildReconciler(shouldTrackSideEffects) {
         warnOnFunctionType();
       }
     }
-    if (typeof newChild === 'undefined') {
+    if (typeof newChild === 'undefined' && !isUnkeyedTopLevelFragment) {
       // If the new child is undefined, and the return fiber is a composite
       // component, throw an error. If Fiber return types are disabled,
       // we already threw above.