Nullness bugfix :: could not assign null to member val property (#17845)

Author: T-Gro

docs/release-notes/.FSharp.Compiler.Service/9.0.200.md

@@ -5,6 +5,7 @@
 * Fix extension methods support for non-reference system assemblies ([PR #17799](https://github.com/dotnet/fsharp/pull/17799))
 * Ensure `frameworkTcImportsCache` mutations are thread-safe. ([PR #17795](https://github.com/dotnet/fsharp/pull/17795))
 * Fix concurrency issue in `ILPreTypeDefImpl`  ([PR #17812](https://github.com/dotnet/fsharp/pull/17812))
+* Fix nullness inference for member val and other OO scenarios  ([PR #17845](https://github.com/dotnet/fsharp/pull/17845))
 
 ### Added
 

src/Compiler/TypedTree/TypedTree.fs

@@ -4358,6 +4358,12 @@ type NullnessVar() =
 
     member nv.IsSolved = solution.IsSome
 
+    member nv.IsFullySolved = 
+        match solution with
+        | None -> false
+        | Some (Nullness.Known _) -> true
+        | Some (Nullness.Variable v) -> v.IsFullySolved
+
     member nv.Set(nullness) = 
        assert (not nv.IsSolved) 
        solution <- Some nullness

src/Compiler/TypedTree/TypedTree.fsi

@@ -3100,6 +3100,7 @@ type NullnessVar =
     member Evaluate: unit -> NullnessInfo
     member TryEvaluate: unit -> NullnessInfo voption
     member IsSolved: bool
+    member IsFullySolved: bool
     member Set: Nullness -> unit
     member Unset: unit -> unit
     member Solution: Nullness

src/Compiler/TypedTree/TypedTreeBasics.fs

@@ -284,7 +284,7 @@ let tryAddNullnessToTy nullnessNew (ty:TType) =
 let addNullnessToTy (nullness: Nullness) (ty:TType) =
     match nullness with
     | Nullness.Known NullnessInfo.WithoutNull -> ty
-    | Nullness.Variable nv when nv.IsSolved && nv.Evaluate() = NullnessInfo.WithoutNull -> ty
+    | Nullness.Variable nv when nv.IsFullySolved && nv.TryEvaluate() = ValueSome NullnessInfo.WithoutNull -> ty
     | _ -> 
     match ty with
     | TType_var (tp, nullnessOrig) -> TType_var (tp, combineNullness nullnessOrig nullness)

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

@@ -144,6 +144,21 @@ let nonStrictFunc(x:string | null) = strictFunc(x)
     |> shouldFail
     |> withDiagnostics [
         Error 3261, Line 4, Col 49, Line 4, Col 50, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."]
+ 
+[<Fact>]
+let ``Can have nullable prop of same type T within a custom type T``() =
+    FSharp """
+module MyLib
+type T () =
+    let mutable v : T | null = null
+    member val P : T | null = null with get, set
+    member this.M() =
+        v <- null
+        this.P <- null
+    """
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
 
 [<Theory>]
 [<InlineData("fileExists(path)")>]