Always trigger componentWillUnmount in StrictMode (#26842)

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## Summary

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Explain the **motivation** for making this change. What existing problem
does the pull request solve?
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In StrictMode, React currently only triggers `componentWillUnmount` if
`componentDidMount` is defined. This would miss detecting issues like
initializing resources in constructor or componentWillMount, for
example:
```
class Component {
  constructor() {
     this._subscriptions = new Subscriptions();
  }
  componentWillUnmount() {
     this._subscriptions.reset();
  }
}
```

The PR makes `componentWillUnmount` always run in StrictMode.

## How did you test this change?

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`yarn test ci`

DiffTrain build for [8110222](8110222)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-018c58c9c65452cff25aaf1f38f78a9b90d8e5c1
+811022232efed62a3c943701bc99d18655bc78b3

compiled/facebook-www/React-dev.modern.js

@@ -27,7 +27,7 @@ if (
 }
           "use strict";
 
-var ReactVersion = "18.3.0-www-modern-d087d75f";
+var ReactVersion = "18.3.0-www-modern-a6d62651";
 
 // ATTENTION
 // When adding new symbols to this file,

compiled/facebook-www/React-prod.modern.js

@@ -641,4 +641,4 @@ exports.useSyncExternalStore = function (
   );
 };
 exports.useTransition = useTransition;
-exports.version = "18.3.0-www-modern-fb2ee8b7";
+exports.version = "18.3.0-www-modern-916dbaab";

compiled/facebook-www/ReactART-dev.classic.js

@@ -69,7 +69,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = "18.3.0-www-classic-603b8e54";
+var ReactVersion = "18.3.0-www-classic-3fed7141";
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -12590,13 +12590,11 @@ function mountClassInstance(workInProgress, ctor, newProps, renderLanes) {
   }
 
   if (typeof instance.componentDidMount === "function") {
-    var fiberFlags = Update | LayoutStatic;
-
-    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-      fiberFlags |= MountLayoutDev;
-    }
+    workInProgress.flags |= Update | LayoutStatic;
+  }
 
-    workInProgress.flags |= fiberFlags;
+  if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+    workInProgress.flags |= MountLayoutDev;
   }
 }
 
@@ -12658,13 +12656,11 @@ function resumeMountClassInstance(workInProgress, ctor, newProps, renderLanes) {
     // If an update was already in progress, we should schedule an Update
     // effect even though we're bailing out, so that cWU/cDU are called.
     if (typeof instance.componentDidMount === "function") {
-      var fiberFlags = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        fiberFlags |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= fiberFlags;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     }
 
     return false;
@@ -12710,25 +12706,21 @@ function resumeMountClassInstance(workInProgress, ctor, newProps, renderLanes) {
     }
 
     if (typeof instance.componentDidMount === "function") {
-      var _fiberFlags = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        _fiberFlags |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= _fiberFlags;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     }
   } else {
     // If an update was already in progress, we should schedule an Update
     // effect even though we're bailing out, so that cWU/cDU are called.
     if (typeof instance.componentDidMount === "function") {
-      var _fiberFlags2 = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        _fiberFlags2 |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= _fiberFlags2;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     } // If shouldComponentUpdate returned false, we should still update the
     // memoized state to indicate that this work can be reused.
 
@@ -23789,10 +23781,12 @@ function invokeLayoutEffectMountInDEV(fiber) {
       case ClassComponent: {
         var instance = fiber.stateNode;
 
-        try {
-          instance.componentDidMount();
-        } catch (error) {
-          captureCommitPhaseError(fiber, fiber.return, error);
+        if (typeof instance.componentDidMount === "function") {
+          try {
+            instance.componentDidMount();
+          } catch (error) {
+            captureCommitPhaseError(fiber, fiber.return, error);
+          }
         }
 
         break;

compiled/facebook-www/ReactART-dev.modern.js

@@ -69,7 +69,7 @@ function _assertThisInitialized(self) {
   return self;
 }
 
-var ReactVersion = "18.3.0-www-modern-d087d75f";
+var ReactVersion = "18.3.0-www-modern-a6d62651";
 
 var LegacyRoot = 0;
 var ConcurrentRoot = 1;
@@ -12322,13 +12322,11 @@ function mountClassInstance(workInProgress, ctor, newProps, renderLanes) {
   }
 
   if (typeof instance.componentDidMount === "function") {
-    var fiberFlags = Update | LayoutStatic;
-
-    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-      fiberFlags |= MountLayoutDev;
-    }
+    workInProgress.flags |= Update | LayoutStatic;
+  }
 
-    workInProgress.flags |= fiberFlags;
+  if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+    workInProgress.flags |= MountLayoutDev;
   }
 }
 
@@ -12383,13 +12381,11 @@ function resumeMountClassInstance(workInProgress, ctor, newProps, renderLanes) {
     // If an update was already in progress, we should schedule an Update
     // effect even though we're bailing out, so that cWU/cDU are called.
     if (typeof instance.componentDidMount === "function") {
-      var fiberFlags = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        fiberFlags |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= fiberFlags;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     }
 
     return false;
@@ -12435,25 +12431,21 @@ function resumeMountClassInstance(workInProgress, ctor, newProps, renderLanes) {
     }
 
     if (typeof instance.componentDidMount === "function") {
-      var _fiberFlags = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        _fiberFlags |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= _fiberFlags;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     }
   } else {
     // If an update was already in progress, we should schedule an Update
     // effect even though we're bailing out, so that cWU/cDU are called.
     if (typeof instance.componentDidMount === "function") {
-      var _fiberFlags2 = Update | LayoutStatic;
-
-      if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
-        _fiberFlags2 |= MountLayoutDev;
-      }
+      workInProgress.flags |= Update | LayoutStatic;
+    }
 
-      workInProgress.flags |= _fiberFlags2;
+    if ((workInProgress.mode & StrictEffectsMode) !== NoMode) {
+      workInProgress.flags |= MountLayoutDev;
     } // If shouldComponentUpdate returned false, we should still update the
     // memoized state to indicate that this work can be reused.
 
@@ -23454,10 +23446,12 @@ function invokeLayoutEffectMountInDEV(fiber) {
       case ClassComponent: {
         var instance = fiber.stateNode;
 
-        try {
-          instance.componentDidMount();
-        } catch (error) {
-          captureCommitPhaseError(fiber, fiber.return, error);
+        if (typeof instance.componentDidMount === "function") {
+          try {
+            instance.componentDidMount();
+          } catch (error) {
+            captureCommitPhaseError(fiber, fiber.return, error);
+          }
         }
 
         break;