Restrict effect return type to a function or nothing

We already warn in dev if the wrong type is returned. This updates the
Flow type.

Author: acdlite

packages/react-debug-tools/src/ReactDebugHooks.js

@@ -141,7 +141,7 @@ function useRef<T>(initialValue: T): {current: T} {
 }
 
 function useMutationEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   nextHook();
@@ -153,7 +153,7 @@ function useMutationEffect(
 }
 
 function useLayoutEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   nextHook();
@@ -165,7 +165,7 @@ function useLayoutEffect(
 }
 
 function useEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   nextHook();

packages/react-dom/src/server/ReactPartialRendererHooks.js

@@ -264,7 +264,7 @@ function useRef<T>(initialValue: T): {current: T} {
 }
 
 function useMutationEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ) {
   warning(
@@ -278,7 +278,7 @@ function useMutationEffect(
 }
 
 export function useLayoutEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ) {
   warning(

packages/react-reconciler/src/ReactFiberHooks.js

@@ -62,7 +62,7 @@ export type Hook = {
 
 type Effect = {
   tag: HookEffectTag,
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   destroy: (() => mixed) | null,
   inputs: Array<mixed>,
   next: Effect,
@@ -518,7 +518,7 @@ export function useRef<T>(initialValue: T): {current: T} {
 }
 
 export function useMutationEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   useEffectImpl(
@@ -530,14 +530,14 @@ export function useMutationEffect(
 }
 
 export function useLayoutEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   useEffectImpl(UpdateEffect, UnmountMutation | MountLayout, create, inputs);
 }
 
 export function useEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ): void {
   useEffectImpl(

packages/react/src/ReactHooks.js

@@ -71,23 +71,23 @@ export function useRef<T>(initialValue: T): {current: T} {
 }
 
 export function useEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ) {
   const dispatcher = resolveDispatcher();
   return dispatcher.useEffect(create, inputs);
 }
 
 export function useMutationEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ) {
   const dispatcher = resolveDispatcher();
   return dispatcher.useMutationEffect(create, inputs);
 }
 
 export function useLayoutEffect(
-  create: () => mixed,
+  create: () => (() => mixed) | null | void,
   inputs: Array<mixed> | void | null,
 ) {
   const dispatcher = resolveDispatcher();