Nullness :: Bugfix for internal error when processing System.Nullable with nesting in C#

docs/release-notes/.FSharp.Compiler.Service/9.0.100.md

@@ -18,8 +18,10 @@
 * Fix IsUnionCaseTester throwing for non-methods/properties [#17301](https://github.com/dotnet/fsharp/pull/17634)
 * Consider `open type` used when the type is an enum and any of the enum cases is used unqualified. ([PR #17628](https://github.com/dotnet/fsharp/pull/17628))
 * Guard for possible StackOverflowException when typechecking non recursive modules and namespaces ([PR #17654](https://github.com/dotnet/fsharp/pull/17654))
+* Nullable - fix for processing System.Nullable types with nesting ([PR #17736](https://github.com/dotnet/fsharp/pull/17736))
 * Fixes for the optimization of simple mappings in array and list comprehensions. ([Issue #17708](https://github.com/dotnet/fsharp/issues/17708), [PR #17711](https://github.com/dotnet/fsharp/pull/17711))
 
+
 ### Added
 
 * Support for nullable reference types ([PR #15181](https://github.com/dotnet/fsharp/pull/15181))

src/Compiler/Checking/import.fs

@@ -273,12 +273,18 @@ For value types, a value is passed even though it is always 0
 
             member this.Advance() = {Data = this.Data; Idx = this.Idx + 1}
 
+    let inline isSystemNullable (tspec:ILTypeSpec) = 
+        match tspec.Name,tspec.Enclosing with
+        | "Nullable`1",["System"] -> true
+        | "System.Nullable`1",[] -> true
+        | _ -> false
+
     let inline evaluateFirstOrderNullnessAndAdvance (ilt:ILType) (flags:NullableFlags) = 
         match ilt with
         | ILType.Value tspec when tspec.GenericArgs.IsEmpty -> KnownWithoutNull, flags
         // System.Nullable is special-cased in C# spec for nullness metadata.
         // You CAN assign 'null' to it, and when boxed, it CAN be boxed to 'null'.
-        | ILType.Value tspec when tspec.Name = "Nullable`1" && tspec.Enclosing = ["System"] -> KnownWithoutNull, flags
+        | ILType.Value tspec when isSystemNullable tspec -> KnownWithoutNull, flags
         | ILType.Value _  -> KnownWithoutNull, flags.Advance()
         | _ -> flags.GetNullness(), flags.Advance()
 

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

@@ -225,4 +225,39 @@ let ``Consumption of nullable C# - no generics, just strings in methods and fiel
             Error 3261, Line 25, Col 85, Line 25, Col 97, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
             Error 3261, Line 28, Col 99, Line 28, Col 111, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
             Error 3261, Line 30, Col 97, Line 30, Col 109, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."]
+
+
+[<FactForNETCOREAPP>]
+let ``Regression 17701 - Nullable value type with nested generics`` () =
+    let csharpLib =
+        CSharp """
+using System;
+using System.Collections.Immutable;
+#nullable enable
+namespace Nullables;
+public class NullableClass {
+    public static ImmutableArray<string?>? nullableImmArrayOfStrings;
+    public static ImmutableArray<string>? nullableImmArrayOfNotNullStrings;
+}""" |> withName "csNullableLib"
+     |> withCSharpLanguageVersionPreview
+
+    FSharp """module FSNullable
+open Nullables
+
+let nullablestrNoParams = NullableClass.nullableImmArrayOfStrings
+let toOption = NullableClass.nullableImmArrayOfStrings |> Option.ofNullable
+let firstString = (toOption.Value |> Seq.head)
+let lengthOfIt = firstString.Length
+
+let theOtherOne = NullableClass.nullableImmArrayOfNotNullStrings
+    """        
+    |> asLibrary
+    |> withReferences [csharpLib]
+    |> withStrictNullness
+    |> withLangVersionPreview
+    |> compile
+    |> shouldFail
+    |> withDiagnostics 
+                [Error 3261, Line 7, Col 18, Line 7, Col 36, "Nullness warning: The types 'string' and 'string | null' do not have compatible nullability."]
+