Fix AttributeUsage.AllowMultiple not inherited for C#-defined attributes

docs/release-notes/.FSharp.Compiler.Service/10.0.300.md

@@ -14,6 +14,7 @@
 * Fix FS0229 B-stream misalignment when reading metadata from assemblies compiled with LangVersion < 9.0, introduced by [#17706](https://github.com/dotnet/fsharp/pull/17706). ([PR #19260](https://github.com/dotnet/fsharp/pull/19260))
 * Fix FS3356 false positive for instance extension members with same name on different types, introduced by [#18821](https://github.com/dotnet/fsharp/pull/18821). ([PR #19260](https://github.com/dotnet/fsharp/pull/19260))
 * F# Scripts: Fix default reference paths resolving when an SDK directory is specified. ([PR #19270](https://github.com/dotnet/fsharp/pull/19270))
+* Fix `AttributeUsage.AllowMultiple` not being inherited for attributes subclassed in C#. ([Issue #17107](https://github.com/dotnet/fsharp/issues/17107), [PR #19315](https://github.com/dotnet/fsharp/pull/19315))
 
 ### Added
 * FSharpType: add ImportILType ([PR #19300](https://github.com/dotnet/fsharp/pull/19300))

src/Compiler/Checking/PostInferenceChecks.fs

@@ -2037,7 +2037,16 @@ and CheckAttribs cenv env (attribs: Attribs) =
         |> Seq.map fst
         |> Seq.toList
         // Filter for allowMultiple = false
-        |> List.filter (fun (tcref, _, m) -> TryFindAttributeUsageAttribute cenv.g m tcref <> Some true)
+        |> List.filter (fun (tcref, _, m) ->
+            let getSuper (tcref: TyconRef) =
+                let ty = generalizedTyconRef cenv.g tcref
+                match GetSuperTypeOfType cenv.g cenv.amap m ty with
+                | Some superTy ->
+                    match tryTcrefOfAppTy cenv.g superTy with
+                    | ValueSome sup -> Some sup
+                    | ValueNone -> None
+                | None -> None
+            TryFindAttributeUsageAttribute cenv.g m getSuper tcref <> Some true)
 
     if cenv.reportErrors then
        for tcref, _, m in duplicates do

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -3489,12 +3489,12 @@ let superOfTycon (g: TcGlobals) (tycon: Tycon) =
     | None -> g.obj_ty_noNulls 
     | Some ty -> ty 
 
-/// walk a TyconRef's inheritance tree, yielding any parent types as an array
-let supersOfTyconRef (tcref: TyconRef) =
+/// Walk a TyconRef's inheritance tree using the provided super-type resolver, yielding parent types as an array.
+let supersOfTyconRefWith (getSuper: TyconRef -> TyconRef option) (tcref: TyconRef) =
     tcref |> Array.unfold (fun tcref ->
-        match tcref.TypeContents.tcaug_super with
-        | Some (TType_app(sup, _, _)) -> Some(sup, sup)
-        | _ -> None)
+        match getSuper tcref with
+        | Some sup -> Some(sup, sup)
+        | None -> None)
 
 //----------------------------------------------------------------------------
 // Detect attributes
@@ -3629,10 +3629,11 @@ let TryFindTyconRefBoolAttribute g m attribSpec tcref =
                    | [ Some (:? bool as v : obj) ], _ -> Some v 
                    | _ -> None)
 
-/// Try to find the resolved attributeusage for an type by walking its inheritance tree and picking the correct attribute usage value
-let TryFindAttributeUsageAttribute g m tcref =
+/// Try to find the resolved AttributeUsage for a type by walking its inheritance tree and picking the correct attribute usage value.
+/// The getSuper function is used to resolve the super-type of each type in the chain, allowing correct handling of both F# and IL-imported types.
+let TryFindAttributeUsageAttribute g m (getSuper: TyconRef -> TyconRef option) tcref =
     [| yield tcref
-       yield! supersOfTyconRef tcref |]
+       yield! supersOfTyconRefWith getSuper tcref |]
     |> Array.tryPick (fun tcref ->
         TryBindTyconRefAttribute g m g.attrib_AttributeUsageAttribute tcref
                 (fun (_, named) -> named |> List.tryPick (function "AllowMultiple", _, _, ILAttribElem.Bool res -> Some res | _ -> None))

src/Compiler/TypedTree/TypedTreeOps.fsi

@@ -2400,8 +2400,9 @@ val TyconRefHasAttribute: TcGlobals -> range -> BuiltinAttribInfo -> TyconRef ->
 /// Try to find an attribute with a specific full name on a type definition
 val TyconRefHasAttributeByName: range -> string -> TyconRef -> bool
 
-/// Try to find the AttributeUsage attribute, looking for the value of the AllowMultiple named parameter
-val TryFindAttributeUsageAttribute: TcGlobals -> range -> TyconRef -> bool option
+/// Try to find the AttributeUsage attribute, looking for the value of the AllowMultiple named parameter.
+/// The getSuper function is used to walk the inheritance chain for both F# and IL-imported types.
+val TryFindAttributeUsageAttribute: TcGlobals -> range -> (TyconRef -> TyconRef option) -> TyconRef -> bool option
 
 #if !NO_TYPEPROVIDERS
 /// returns Some(assemblyName) for success

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

@@ -77,3 +77,114 @@ type C() =
         |> withReferences [csharpBaseClass]
         |> compile
         |> shouldSucceed
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``C# attribute subclass inherits AllowMultiple true from base`` () =
+        let csharpLib =
+            CSharp """
+            using System;
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+            public class BaseAttribute : Attribute { }
+            public class ChildAttribute : BaseAttribute { }
+            """ |> withName "csAttrLib"
+
+        FSharp """
+module Test
+
+[<Child; Child>]
+type C() = class end
+        """
+        |> withReferences [csharpLib]
+        |> compile
+        |> shouldSucceed
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``C# attribute subclass inherits AllowMultiple false from base`` () =
+        let csharpLib =
+            CSharp """
+            using System;
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = false)]
+            public class BaseAttribute : Attribute { }
+            public class ChildAttribute : BaseAttribute { }
+            """ |> withName "csAttrLib"
+
+        FSharp """
+module Test
+
+[<Child; Child>]
+type C() = class end
+        """
+        |> withReferences [csharpLib]
+        |> compile
+        |> shouldFail
+        |> withSingleDiagnostic (Error 429, Line 4, Col 10, Line 4, Col 15, "The attribute type 'ChildAttribute' has 'AllowMultiple=false'. Multiple instances of this attribute cannot be attached to a single language element.")
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``C# attribute multi-level inheritance inherits AllowMultiple true`` () =
+        let csharpLib =
+            CSharp """
+            using System;
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+            public class BaseAttribute : Attribute { }
+            public class MiddleAttribute : BaseAttribute { }
+            public class LeafAttribute : MiddleAttribute { }
+            """ |> withName "csAttrLib"
+
+        FSharp """
+module Test
+
+[<Leaf; Leaf>]
+type C() = class end
+        """
+        |> withReferences [csharpLib]
+        |> compile
+        |> shouldSucceed
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``C# attribute subclass with own AttributeUsage overrides base AllowMultiple`` () =
+        let csharpLib =
+            CSharp """
+            using System;
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+            public class BaseAttribute : Attribute { }
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = false)]
+            public class ChildAttribute : BaseAttribute { }
+            """ |> withName "csAttrLib"
+
+        FSharp """
+module Test
+
+[<Child; Child>]
+type C() = class end
+        """
+        |> withReferences [csharpLib]
+        |> compile
+        |> shouldFail
+        |> withSingleDiagnostic (Error 429, Line 4, Col 10, Line 4, Col 15, "The attribute type 'ChildAttribute' has 'AllowMultiple=false'. Multiple instances of this attribute cannot be attached to a single language element.")
+
+    [<FSharp.Test.FactForNETCOREAPP>]
+    let ``F# attribute subclass of C# base inherits AllowMultiple true`` () =
+        let csharpLib =
+            CSharp """
+            using System;
+
+            [AttributeUsage(AttributeTargets.All, AllowMultiple = true)]
+            public class BaseAttribute : Attribute { }
+            """ |> withName "csAttrLib"
+
+        FSharp """
+module Test
+
+type ChildAttribute() = inherit BaseAttribute()
+
+[<Child; Child>]
+type C() = class end
+        """
+        |> withReferences [csharpLib]
+        |> compile
+        |> shouldSucceed