Fix false positive context warning when using an old React (#13850)

Author: gaearon

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -1118,13 +1118,18 @@ function updateContextConsumer(
   // in DEV mode if this property exists or not and warn if it does not.
   if (__DEV__) {
     if ((context: any)._context === undefined) {
-      if (!hasWarnedAboutUsingContextAsConsumer) {
-        hasWarnedAboutUsingContextAsConsumer = true;
-        warning(
-          false,
-          'Rendering <Context> directly is not supported and will be removed in ' +
-            'a future major release. Did you mean to render <Context.Consumer> instead?',
-        );
+      // This may be because it's a Context (rather than a Consumer).
+      // Or it may be because it's older React where they're the same thing.
+      // We only want to warn if we're sure it's a new React.
+      if (context !== context.Consumer) {
+        if (!hasWarnedAboutUsingContextAsConsumer) {
+          hasWarnedAboutUsingContextAsConsumer = true;
+          warning(
+            false,
+            'Rendering <Context> directly is not supported and will be removed in ' +
+              'a future major release. Did you mean to render <Context.Consumer> instead?',
+          );
+        }
       }
     } else {
       context = (context: any)._context;

packages/react-reconciler/src/__tests__/ReactNewContext-test.internal.js

@@ -1584,6 +1584,28 @@ Context fuzz tester error! Copy and paste the following line into the test suite
     );
   });
 
+  // False positive regression test.
+  it('should not warn when using Consumer from React < 16.6 with newer renderer', () => {
+    const BarContext = React.createContext({value: 'bar-initial'});
+    // React 16.5 and earlier didn't have a separate object.
+    BarContext.Consumer = BarContext;
+
+    function Component() {
+      return (
+        <React.Fragment>
+          <BarContext.Provider value={{value: 'bar-updated'}}>
+            <BarContext.Consumer>
+              {({value}) => <div actual={value} expected="bar-updated" />}
+            </BarContext.Consumer>
+          </BarContext.Provider>
+        </React.Fragment>
+      );
+    }
+
+    ReactNoop.render(<Component />);
+    ReactNoop.flush();
+  });
+
   it('should warn with an error message when using nested context consumers in DEV', () => {
     const BarContext = React.createContext({value: 'bar-initial'});
     const BarConsumer = BarContext;