Allow intrinsic extensions for static members in interfaces (dotnet#1…

…6157)

Allow intrinsic extensions for static members in interfaces

src/Compiler/Checking/CheckDeclarations.fs

@@ -4069,6 +4069,10 @@ module TcDeclarations =
             tcref.Deref.IsFSharpDelegateTycon ||
             tcref.Deref.IsFSharpEnumTycon
 
+        let isDelegateOrEnum = 
+            tcref.Deref.IsFSharpDelegateTycon ||
+            tcref.Deref.IsFSharpEnumTycon
+
         let reqTypars = tcref.Typars m
 
         // Member definitions are intrinsic (added directly to the type) if:
@@ -4104,7 +4108,7 @@ module TcDeclarations =
                 // Note we return 'reqTypars' for intrinsic extensions since we may only have given warnings
                 IntrinsicExtensionBinding, reqTypars
             else 
-                if isInSameModuleOrNamespace && isInterfaceOrDelegateOrEnum then 
+                if isInSameModuleOrNamespace && isDelegateOrEnum then 
                     errorR(Error(FSComp.SR.tcMembersThatExtendInterfaceMustBePlacedInSeparateModule(), tcref.Range))
                 if nReqTypars <> synTypars.Length then 
                     error(Error(FSComp.SR.tcDeclaredTypeParametersForExtensionDoNotMatchOriginal(tcref.DisplayNameWithStaticParametersAndUnderscoreTypars), m))

tests/FSharp.Compiler.ComponentTests/Language/InterfaceTests.fs

@@ -62,6 +62,24 @@ type I<'T> =
         (Error 3350, Line 4, Col 19, Line 4, Col 23, "Feature 'Static members in interfaces' is not available in F# 7.0. Please use language version 8.0 or greater.")
     ]
 
+[<Fact>]
+let ``Concrete static members are allowed in interfaces as intrinsics in lang preview``() =
+    FSharp $"""
+[<Interface>]
+type I<'T> = 
+    static member Prop = Unchecked.defaultof<'T>
+type I<'T> with
+    static member Echo (x: 'T) = x
+
+if I<int>.Echo 42 <> 42 || I<int>.Prop <> 0 || not (isNull I<string>.Prop) then
+    failwith "failed"
+    """
+    |> withLangVersion80
+    |> asExe
+    |> compileAndRun
+    |> shouldSucceed
+
+
 [<Fact>]
 let ``Interface with concrete static members can be implemented in lang preview``() =
     FSharp $"""