Edge case: forbid readContext() during eager reducer

packages/react-reconciler/src/ReactFiberHooks.js

@@ -887,7 +887,13 @@ function dispatchAction<S, A>(
       if (eagerReducer !== null) {
         try {
           const currentState: S = (queue.eagerState: any);
+          // Temporarily clear to forbid calling Hooks in a reducer.
+          let maybeFiber = currentlyRenderingFiber; // Note: likely null now unlike `fiber`
+          currentlyRenderingFiber = null;
+          stashContextDependencies();
           const eagerState = eagerReducer(currentState, action);
+          currentlyRenderingFiber = maybeFiber;
+          unstashContextDependencies();
           // Stash the eagerly computed state, and the reducer used to compute
           // it, on the update object. If the reducer hasn't changed by the
           // time we enter the render phase, then the eager state can be used

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

@@ -778,6 +778,41 @@ describe('ReactHooks', () => {
     );
   });
 
+  // Edge case.
+  it('throws when reading context inside eager useReducer', () => {
+    const {useState, createContext} = React;
+    const ThemeContext = createContext('light');
+
+    const ReactCurrentDispatcher =
+      React.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED
+        .ReactCurrentDispatcher;
+
+    let _setState;
+    function Fn() {
+      const [_, setState] = useState(0);
+      _setState = setState;
+      return null;
+    }
+
+    class Cls extends React.Component {
+      render() {
+        _setState(() => {
+          ReactCurrentDispatcher.current.readContext(ThemeContext);
+        });
+        return null;
+      }
+    }
+
+    expect(() =>
+      ReactTestRenderer.create(
+        <React.Fragment>
+          <Fn />
+          <Cls />
+        </React.Fragment>,
+      ),
+    ).toThrow('Context can only be read while React is rendering');
+  });
+
   it('throws when calling hooks inside useReducer', () => {
     const {useReducer, useRef} = React;
     function App() {