Eenable LanguageFeature.UnmanagedConstraintCsharpInterop for F# 10.0 (dotnet#18737)

Author: Copilot

src/Compiler/Facilities/LanguageFeatures.fs

@@ -236,10 +236,10 @@ type LanguageVersion(versionText) =
                 LanguageFeature.BetterAnonymousRecordParsing, languageVersion100
                 LanguageFeature.ScopedNowarn, languageVersion100
                 LanguageFeature.AllowTypedLetOrUseBang, languageVersion100
+                LanguageFeature.UnmanagedConstraintCsharpInterop, languageVersion100
                 LanguageFeature.AllowAccessModifiersToAutoPropertiesGettersAndSetters, languageVersion100
 
                 // F# preview (still preview in 10.0)
-                LanguageFeature.UnmanagedConstraintCsharpInterop, previewVersion // not enabled because: https://github.com/dotnet/fsharp/issues/17509
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
             ]
 

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -9971,6 +9971,7 @@ let isCompiledOrWitnessPassingConstraint (g: TcGlobals) cx =
       | TyparConstraint.IsNonNullableStruct _ 
       | TyparConstraint.IsReferenceType _
       | TyparConstraint.RequiresDefaultConstructor _
+      | TyparConstraint.IsUnmanaged _ //  implies "struct" and also causes a modreq
       | TyparConstraint.CoercesTo _ -> true
       | TyparConstraint.MayResolveMember _ when g.langVersion.SupportsFeature LanguageFeature.WitnessPassing -> true
       | _ -> false

tests/FSharp.Compiler.ComponentTests/Conformance/Constraints/Unmanaged.fs

@@ -471,21 +471,72 @@ printf "%s" (CsharpStruct<int>.Hi<MultiCaseUnion>())
     IL_0000:  ret
   } """]
 
-    [<Fact>]
-    let ``FSharp does not generate modreq for VBNET to consume in v7`` () = 
+    [<FactForNETCOREAPP>]
+    let ``FSharp generates modreq for CSharp to consume in v9`` () = 
         Fsx "let testMyFunction (x: 'TUnmanaged when 'TUnmanaged : unmanaged) = ()"
-        |> withLangVersion70
+        |> withLangVersion10
         |> compile
         |> shouldSucceed
         |> verifyIL ["""
-      .method public static void  testMyFunction<TUnmanaged>(!!TUnmanaged x) cil managed
+      .method public static void  testMyFunction<valuetype (class [runtime]System.ValueType modreq([runtime]System.Runtime.InteropServices.UnmanagedType)) TUnmanaged>(!!TUnmanaged x) cil managed
   {
+    .param type TUnmanaged 
+      .custom instance void [runtime]System.Runtime.CompilerServices.IsUnmanagedAttribute::.ctor() = ( 01 00 00 00 ) 
     
     .maxstack  8
     IL_0000:  ret
   } """]
 
 
+    [<Fact>]
+    let ``Unmanaged constraint in lambda reproduces issue 17509`` () = 
+        // This test reproduces the issue https://github.com/dotnet/fsharp/issues/17509
+        // When UnmanagedConstraintCsharpInterop is enabled, it generates invalid IL
+        // causing a TypeLoadException at runtime
+        Fsx """
+open System
+
+let printTypeConstraintsNative<'T when 'T : unmanaged> () = printf $"Hello: {typeof<'T>.FullName} is unmanaged"
+
+let Main() =
+    let func (x:int) : 'T when 'T : unmanaged = Unchecked.defaultof<'T>
+    let initFinite = Seq.init<nativeint> 3 func
+    printf "%A" initFinite
+ 
+printTypeConstraintsNative<nativeint>()
+Main()
+        """
+        |> withLangVersionPreview
+        |> asExe
+        |> compileAndRun
+        |> shouldSucceed
+        |> verifyOutput "Hello: System.IntPtr is unmanagedseq [0n; 0n; 0n]"
+
+    [<FactForNETCOREAPP>]
+    let ``Unmanaged constraint in lambda generates invalid IL for Specialize method with preview version`` () = 
+        Fsx """
+let Main() =
+    let func (x:int) : 'T when 'T : unmanaged = Unchecked.defaultof<'T>
+    let initFinite = Seq.init<nativeint> 3 func
+    printfn "%A" initFinite
+Main()
+        """
+        |> withLangVersionPreview
+        |> asExe
+        |> compile
+        |> shouldSucceed
+        |> verifyIL ["""
+    .method assembly strict virtual instance class [FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<int32,!!T> DirectInvoke<valuetype (class [runtime]System.ValueType modreq([runtime]System.Runtime.InteropServices.UnmanagedType)) T>() cil managed
+    {
+      .param type T 
+        .custom instance void [runtime]System.Runtime.CompilerServices.IsUnmanagedAttribute::.ctor() = ( 01 00 00 00 ) 
+      
+      .maxstack  8
+      IL_0000:  ldsfld     class Test/'func@3-1'<!0> class Test/'func@3-1'<!!T>::@_instance
+      IL_0005:  ret
+    } """]
+
+
     [<Fact>]
     let ``C# can consume F#-defined struct with unmanaged constraint - valid`` () =
         let fsharpLib =