Nullness - option<> must be indicated as nullable in IL for C# consumers

src/Compiler/CodeGen/IlxGenSupport.fs

@@ -407,6 +407,8 @@ let rec GetNullnessFromTType (g: TcGlobals) ty =
                 else if isValueType then
                     // Generic value type: 0, followed by the representation of the type arguments in order including containing types
                     yield NullnessInfo.AmbivalentToNull
+                else if IsUnionTypeWithNullAsTrueValue g tcref.Deref || TypeHasAllowNull tcref g FSharp.Compiler.Text.Range.Zero then
+                    yield NullnessInfo.WithNull
                 else
                     // Reference type: the nullability (0, 1, or 2), followed by the representation of the type arguments in order including containing types
                     yield nullness.Evaluate()

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -9185,16 +9185,18 @@ let reqTyForArgumentNullnessInference g actualTy reqTy =
         changeWithNullReqTyToVariable g reqTy       
     | _ -> reqTy
 
+let TypeHasAllowNull (tcref:TyconRef) g m =
+    not tcref.IsStructOrEnumTycon &&
+    not (isByrefLikeTyconRef g m tcref) && 
+    (TryFindTyconRefBoolAttribute g m g.attrib_AllowNullLiteralAttribute tcref = Some true)
+
 /// The new logic about whether a type admits the use of 'null' as a value.
 let TypeNullIsExtraValueNew g m ty = 
     let sty = stripTyparEqns ty
 
     // Check if the type has AllowNullLiteral
     (match tryTcrefOfAppTy g sty with 
-     | ValueSome tcref -> 
-        not tcref.IsStructOrEnumTycon &&
-        not (isByrefLikeTyconRef g m tcref) && 
-        (TryFindTyconRefBoolAttribute g m g.attrib_AllowNullLiteralAttribute tcref = Some true)
+     | ValueSome tcref -> TypeHasAllowNull tcref g m
      | _ -> false) 
     ||
     // Check if the type has a nullness annotation

src/Compiler/TypedTree/TypedTreeOps.fsi

@@ -1809,6 +1809,8 @@ val TypeNullIsTrueValue: TcGlobals -> TType -> bool
 
 val TypeNullIsExtraValue: TcGlobals -> range -> TType -> bool
 
+val TypeHasAllowNull: TyconRef -> TcGlobals -> range -> bool
+
 val TypeNullIsExtraValueNew: TcGlobals -> range -> TType -> bool
 
 val TypeNullNever: TcGlobals -> TType -> bool

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

@@ -928,4 +928,55 @@ let v3WithNull = f3 (null: obj | null)
               Error 3261, Line 8, Col 14, Line 8, Col 20, "Nullness warning: The type ''a option' uses 'null' as a representation value but a non-null type is expected."
               Error 3261, Line 10, Col 11, Line 10, Col 15, "Nullness warning: The type 'obj' does not support 'null'."
               Error 3261, Line 11, Col 35, Line 11, Col 37, "Nullness warning: The type 'String | null' supports 'null' but a non-null type is expected."
-              Error 3261, Line 13, Col 22, Line 13, Col 38, "Nullness warning: The type 'obj | null' supports 'null' but a non-null type is expected."]
+              Error 3261, Line 13, Col 22, Line 13, Col 38, "Nullness warning: The type 'obj | null' supports 'null' but a non-null type is expected."]
+
+
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Option type detected as null for NullabilityInfoContext`` () =
+
+    Fsx """
+
+open System
+open System.Reflection
+
+type MyClass(StringOrNull: string | null, StringOption: string option, ObjNull: objnull, ObjOrNull: obj | null) =
+    member val StringOrNull = StringOrNull
+    member val StringOption = StringOption
+    member val ObjNull = ObjNull
+    member val ObjOrNull = ObjOrNull
+
+let classType = typeof<MyClass>
+let ctor = classType.GetConstructors() |> Seq.exactlyOne
+let paramInfos = ctor.GetParameters()
+
+
+let nrtContext = NullabilityInfoContext()
+for paramInfo in paramInfos do
+    let nrtInfo = nrtContext.Create(paramInfo)
+    let readState = nrtInfo.ReadState
+    let writeState = nrtInfo.WriteState
+    printfn $"{paramInfo.Name} => {readState} / {writeState}" """
+    |> withNullnessOptions
+    |> compile
+    |> run
+    |> verifyOutputContains [|"StringOption => Nullable / Nullable"|]
+
+[<FSharp.Test.FactForNETCOREAPPAttribute>]
+let ``Option type has WithNull annotation in IL`` () =
+    FSharp """
+module Test
+
+let myOption () : option<string> = None  """
+    |> withNullnessOptions
+    |> compile
+    // The option<> itself is nullable (2), the string inside is not (1)
+    |> verifyIL ["      
+      .method public static class [FSharp.Core]Microsoft.FSharp.Core.FSharpOption`1<string> myOption() cil managed
+      {
+        .param [0]
+        .custom instance void [runtime]System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8[]) = ( 01 00 02 00 00 00 02 01 00 00 ) 
+
+        .maxstack  8
+        IL_0000:  ldnull
+        IL_0001:  ret
+      }"]