Fix nullable Event<'Delegate, 'Args> for INotifyPropertyChanged

Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Author: Copilot

src/FSharp.Core/event.fs

@@ -83,7 +83,7 @@ type EventWrapper<'Delegate, 'Args> = delegate of 'Delegate * objnull * 'Args ->
 type Event<'Delegate, 'Args
     when 'Delegate: delegate<'Args, unit> and 'Delegate :> System.Delegate and 'Delegate: not struct>() =
 
-    let mutable multicast: 'Delegate = Unchecked.defaultof<_>
+    let mutable multicast: 'Delegate | null = Unchecked.defaultof<_>
 
     static let mi, argTypes =
         let instanceBindingFlags =
@@ -146,10 +146,10 @@ type Event<'Delegate, 'Args
                 "<published event>"
           interface IEvent<'Delegate, 'Args> with
               member e.AddHandler(d) =
-                  Atomic.setWith (fun value -> System.Delegate.Combine(value, d) :?> 'Delegate) &multicast
+                  Atomic.setWith (fun value -> System.Delegate.Combine(value, d) :?> ('Delegate | null)) &multicast
 
               member e.RemoveHandler(d) =
-                  Atomic.setWith (fun value -> System.Delegate.Remove(value, d) :?> 'Delegate) &multicast
+                  Atomic.setWith (fun value -> System.Delegate.Remove(value, d) :?> ('Delegate | null)) &multicast
           interface System.IObservable<'Args> with
               member e.Subscribe(observer) =
                   let obj = new EventDelegee<'Args>(observer)
@@ -166,7 +166,7 @@ type Event<'Delegate, 'Args
 
 [<CompiledName("FSharpEvent`1")>]
 type Event<'T> =
-    val mutable multicast: Handler<'T>
+    val mutable multicast: Handler<'T> | null
     new() = { multicast = null }
 
     member x.Trigger(arg: 'T) =
@@ -180,10 +180,10 @@ type Event<'T> =
                 "<published event>"
           interface IEvent<'T> with
               member e.AddHandler(d) =
-                  Atomic.setWith (fun value -> System.Delegate.Combine(value, d) :?> Handler<'T>) &x.multicast
+                  Atomic.setWith (fun value -> System.Delegate.Combine(value, d) :?> Handler<'T> | null) &x.multicast
 
               member e.RemoveHandler(d) =
-                  Atomic.setWith (fun value -> System.Delegate.Remove(value, d) :?> Handler<'T>) &x.multicast
+                  Atomic.setWith (fun value -> System.Delegate.Remove(value, d) :?> Handler<'T> | null) &x.multicast
           interface System.IObservable<'T> with
               member e.Subscribe(observer) =
                   let h = new Handler<_>(fun sender args -> observer.OnNext(args))

tests/FSharp.Compiler.ComponentTests/Language/Nullness/Files/EventNullabilityTest.fs

@@ -0,0 +1,10 @@
+namespace Nullness
+
+open System.ComponentModel
+
+type XViewModel() =
+    let propertyChanged = Event<PropertyChangedEventHandler, PropertyChangedEventArgs>()
+
+    interface INotifyPropertyChanged with
+        [<CLIEvent>]
+        member this.PropertyChanged = propertyChanged.Publish

tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs

@@ -1540,4 +1540,21 @@ let main _ =
     |> compile
     //|> verifyIL ["abc"]
     |> run
-    |> verifyOutputContains [|"Test true;,1 true,2 true,3 true,4 true,5 true,6 false,7 true,8 false,9 false,10 false,11 false,12 true"|]
+    |> verifyOutputContains [|"Test true;,1 true,2 true,3 true,4 true,5 true,6 false,7 true,8 false,9 false,10 false,11 false,12 true"|]
+
+[<Fact>]
+let ``INotifyPropertyChanged with Event using PropertyChangedEventHandler`` () =
+    FSharp """namespace Nullness
+
+open System.ComponentModel
+
+type XViewModel() =
+    let propertyChanged = Event<PropertyChangedEventHandler, PropertyChangedEventArgs>()
+
+    interface INotifyPropertyChanged with
+        [<CLIEvent>]
+        member this.PropertyChanged = propertyChanged.Publish
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed