Fix #19664: Static extension overload resolution for generic types

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -48,6 +48,7 @@
 * Fix signature generation: type params with special characters missing backtick escaping. ([Issue #19595](https://github.com/dotnet/fsharp/issues/19595), [PR #19609](https://github.com/dotnet/fsharp/pull/19609))
 * Fix internal error when using custom attribute with `[<Optional>]` value type parameter and no `[<DefaultParameterValue>]`. ([Issue #8353](https://github.com/dotnet/fsharp/issues/8353), [PR #19484](https://github.com/dotnet/fsharp/pull/19484))
 * Fix parallel compilation of scripts ([PR #19649](https://github.com/dotnet/fsharp/pull/19649))
+* Fix overload resolution of static member extension on generic types when the call uses explicit type arguments and an intrinsic overload of the same name exists. ([Issue #19664](https://github.com/dotnet/fsharp/issues/19664))
 
 ### Added
 

src/Compiler/Checking/NameResolution.fs

@@ -2874,6 +2874,23 @@ let rec ResolveLongIdentInTypePrim (ncenv: NameResolver) nenv lookupKind (resInf
             | Some(MethodItem msets) when isLookUpExpr ->
                 let minfos = msets |> ExcludeHiddenOfMethInfos g ncenv.amap m
 
+                // If the intrinsic candidate set already disagrees with the strict
+                // instance/static filter (e.g. expression-form `Type<TArg>.Member` is
+                // resolved as a dot-expression with LookupIsInstance.Yes but yields
+                // static intrinsic members), relax the filter for the extension lookup
+                // so static extension members are also considered. See issue #19664.
+                let isAmbivalent =
+                    minfos
+                    |> List.exists (fun minfo ->
+                        match isInstanceFilter with
+                        | LookupIsInstance.Yes -> not minfo.IsInstance
+                        | LookupIsInstance.No -> minfo.IsInstance
+                        | LookupIsInstance.Ambivalent -> true)
+
+                let isInstanceFilter =
+                    if isAmbivalent then LookupIsInstance.Ambivalent
+                    else isInstanceFilter
+
                 // fold the available extension members into the overload resolution
                 let extensionMethInfos = ExtensionMethInfosOfTypeInScope ResultCollectionSettings.AllResults ncenv.InfoReader nenv ad optFilter isInstanceFilter m ty
 

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/MethodResolution/StaticMethodResolution.fs

@@ -0,0 +1,42 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+
+namespace Conformance.BasicGrammarElements
+
+open FSharp.Test.Compiler
+open Xunit
+
+module StaticMethodResolution =
+
+    // Regression test for https://github.com/dotnet/fsharp/issues/19664
+    //
+    // When a static extension method is defined in a *different* [<AutoOpen>] module than
+    // the generic type it extends, and shares its name with an intrinsic static member,
+    // resolving the call via the explicit-type-argument syntax `Type<TArg>.Member(...)`
+    // previously failed with FS0505. The non-generic dotted form `Type.Member(...)`
+    // resolved correctly, so any regression test that omits the explicit type arguments
+    // does not actually exercise the bug. See the discussion at
+    // https://github.com/dotnet/fsharp/issues/19675#issuecomment-4373059900.
+    [<Fact>]
+    let ``Static extension on generic type resolves with explicit type arguments (issue 19664)`` () =
+        Fsx """
+module Extensions =
+
+    type StaticGeneric<'T> =
+        static member Bar() = ()
+        static member Bar(_: int, _: int) = ()
+
+    [<AutoOpen>]
+    module StaticGenericExtensions =
+        type StaticGeneric<'T> with
+            static member Bar(_: int) = ()
+
+module Program =
+    open Extensions
+
+    StaticGeneric<int>.Bar()        // intrinsic, 0 args
+    StaticGeneric<int>.Bar(42)      // regressed: extension, 1 arg, see issue 19664 (FS0505)
+    StaticGeneric<int>.Bar(42, 0)   // intrinsic, 2 args
+        """
+        |> withOptions ["--nowarn:1125"]
+        |> typecheck
+        |> shouldSucceed

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -64,6 +64,7 @@
     <Compile Include="Conformance\BasicGrammarElements\MemberDefinitions\OptionalDefaultParamArgs\OptionalDefaultParamArgs.fs" />
     <Compile Include="Conformance\BasicGrammarElements\MemberDefinitions\OverloadingMembers\OverloadingMembers.fs" />
     <Compile Include="Conformance\BasicGrammarElements\MethodResolution\MethodResolution.fs" />
+    <Compile Include="Conformance\BasicGrammarElements\MethodResolution\StaticMethodResolution.fs" />
     <Compile Include="Conformance\BasicGrammarElements\ModuleAbbreviations\ModuleAbbreviations.fs" />
     <Compile Include="Conformance\BasicGrammarElements\ModuleDefinitions\ModuleDefinitions.fs" />
     <Compile Include="Conformance\BasicGrammarElements\NamespaceDeclGroups\NamespaceDeclGroups.fs" />