diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index 7c659b7775a1e..68a811e3fe088 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -1142,6 +1142,16 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
+  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
+    if (__DEV__) {
+      console.error(
+        'flushSync was called from inside a lifecycle method. React cannot ' +
+          'flush when React is already rendering. Consider moving this call to ' +
+          'a scheduler task or micro task.',
+      );
+    }
+    return fn(a);
+  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1158,17 +1168,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
-      flushSyncCallbacks();
-    } else {
-      if (__DEV__) {
-        console.error(
-          'flushSync was called from inside a lifecycle method. React cannot ' +
-            'flush when React is already rendering. Consider moving this call to ' +
-            'a scheduler task or micro task.',
-        );
-      }
-    }
+    flushSyncCallbacks();
   }
 }
 
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 530646f8a4d29..729e8e6717d05 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -1142,6 +1142,16 @@ export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
 
 export function flushSync<A, R>(fn: A => R, a: A): R {
   const prevExecutionContext = executionContext;
+  if ((prevExecutionContext & (RenderContext | CommitContext)) !== NoContext) {
+    if (__DEV__) {
+      console.error(
+        'flushSync was called from inside a lifecycle method. React cannot ' +
+          'flush when React is already rendering. Consider moving this call to ' +
+          'a scheduler task or micro task.',
+      );
+    }
+    return fn(a);
+  }
   executionContext |= BatchedContext;
 
   const previousPriority = getCurrentUpdatePriority();
@@ -1158,17 +1168,7 @@ export function flushSync<A, R>(fn: A => R, a: A): R {
     // Flush the immediate callbacks that were scheduled during this batch.
     // Note that this will happen even if batchedUpdates is higher up
     // the stack.
-    if ((executionContext & (RenderContext | CommitContext)) === NoContext) {
-      flushSyncCallbacks();
-    } else {
-      if (__DEV__) {
-        console.error(
-          'flushSync was called from inside a lifecycle method. React cannot ' +
-            'flush when React is already rendering. Consider moving this call to ' +
-            'a scheduler task or micro task.',
-        );
-      }
-    }
+    flushSyncCallbacks();
   }
 }
 
diff --git a/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js b/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js
deleted file mode 100644
index f763e44705aaa..0000000000000
--- a/packages/react-reconciler/src/__tests__/ReactFlushSync-test.js
+++ /dev/null
@@ -1,64 +0,0 @@
-let React;
-let ReactNoop;
-let Scheduler;
-let useState;
-let useEffect;
-
-describe('ReactFlushSync', () => {
-  beforeEach(() => {
-    jest.resetModules();
-
-    React = require('react');
-    ReactNoop = require('react-noop-renderer');
-    Scheduler = require('scheduler');
-    useState = React.useState;
-    useEffect = React.useEffect;
-  });
-
-  function Text({text}) {
-    Scheduler.unstable_yieldValue(text);
-    return text;
-  }
-
-  // @gate experimental || !enableSyncDefaultUpdates
-  test('changes priority of updates in useEffect', async () => {
-    function App() {
-      const [syncState, setSyncState] = useState(0);
-      const [state, setState] = useState(0);
-      useEffect(() => {
-        if (syncState !== 1) {
-          setState(1);
-          ReactNoop.flushSync(() => setSyncState(1));
-        }
-      }, [syncState, state]);
-      return <Text text={`${syncState}, ${state}`} />;
-    }
-
-    const root = ReactNoop.createRoot();
-    await ReactNoop.act(async () => {
-      if (gate(flags => flags.enableSyncDefaultUpdates)) {
-        React.unstable_startTransition(() => {
-          root.render(<App />);
-        });
-      } else {
-        root.render(<App />);
-      }
-      // This will yield right before the passive effect fires
-      expect(Scheduler).toFlushUntilNextPaint(['0, 0']);
-
-      // The passive effect will schedule a sync update and a normal update.
-      // They should commit in two separate batches. First the sync one.
-      expect(() => {
-        expect(Scheduler).toFlushUntilNextPaint(['1, 0']);
-      }).toErrorDev('flushSync was called from inside a lifecycle method');
-
-      // The remaining update is not sync
-      ReactNoop.flushSync();
-      expect(Scheduler).toHaveYielded([]);
-
-      // Now flush it.
-      expect(Scheduler).toFlushUntilNextPaint(['1, 1']);
-    });
-    expect(root).toMatchRenderedOutput('1, 1');
-  });
-});