Fix empty array handling in custom attributes: use Object element type instead of erroring

Empty arrays of user-defined types in custom attributes (e.g. [<DefaultValue([||] : A[])>])
no longer produce FS3885. Since no elements need encoding, System.Object is used as the
element type. Non-empty arrays of unencodable types still error correctly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro

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

@@ -19,7 +19,7 @@
 * Fixed how the source ranges of warn directives are reported (as trivia) in the parser output (by not reporting leading spaces). ([Issue #19405](https://github.com/dotnet/fsharp/issues/19405), [PR #19408]((https://github.com/dotnet/fsharp/pull/19408)))
 * Fix UoM value type `ToString()` returning garbage values when `--checknulls+` is enabled, caused by double address-taking in codegen. ([Issue #19435](https://github.com/dotnet/fsharp/issues/19435), [PR #19440](https://github.com/dotnet/fsharp/pull/19440))
 * Fix completion inconsistently showing some obsolete members (fields and events) while hiding others (methods and properties). All obsolete members are now consistently hidden by default. ([Issue #13512](https://github.com/dotnet/fsharp/issues/13512), [PR #19506](https://github.com/dotnet/fsharp/pull/19506))
-* Fix internal error `FS0192: encodeCustomAttrElemType` when using an array of user-defined type as a custom attribute argument. Now reports a proper diagnostic (FS3885). ([Issue #12796](https://github.com/dotnet/fsharp/issues/12796), [PR #19472](https://github.com/dotnet/fsharp/pull/19472))
+* Fix internal error `FS0192: encodeCustomAttrElemType` when using an empty array of user-defined type as a custom attribute argument (e.g. `[<DefaultValue([||] : A[])>]`). Empty arrays of arbitrary types are now encoded using `System.Object` as the element type. ([Issue #12796](https://github.com/dotnet/fsharp/issues/12796), [PR #19472](https://github.com/dotnet/fsharp/pull/19472))
 
 ### Added
 

src/Compiler/CodeGen/IlxGen.fs

@@ -10224,18 +10224,26 @@ and GenAttribArg amap g eenv x (ilArgTy: ILType) =
     | Expr.Op(TOp.Array, [ elemTy ], args, m), _ ->
         let ilElemTy = GenType amap m eenv.tyenv elemTy
 
-        // Validate element type is encodable in custom attribute metadata (ECMA 335).
-        // Only primitive types, enums, string, System.Type, and System.Object are valid.
-        match ilElemTy with
-        | ILType.Boxed tspec when
-            tspec.Name <> "System.String"
-            && tspec.Name <> "System.Object"
-            && tspec.Name <> "System.Type"
-            ->
-            error (Error(FSComp.SR.ilCustomAttrInvalidArrayElemType (tspec.Name), m))
-        | _ -> ()
+        // Check if element type can be encoded in custom attribute metadata (ECMA 335).
+        // Valid element types: primitives, enums, string, System.Type, System.Object.
+        let isUnencodableBoxedElemType =
+            match ilElemTy with
+            | ILType.Boxed tspec when
+                tspec.Name <> "System.String"
+                && tspec.Name <> "System.Object"
+                && tspec.Name <> "System.Type"
+                ->
+                true
+            | _ -> false
 
-        ILAttribElem.Array(ilElemTy, List.map (fun arg -> GenAttribArg amap g eenv arg ilElemTy) args)
+        if isUnencodableBoxedElemType then
+            if args.IsEmpty then
+                // Empty arrays: substitute System.Object as element type since no elements need encoding.
+                ILAttribElem.Array(g.ilg.typ_Object, [])
+            else
+                error (Error(FSComp.SR.ilCustomAttrInvalidArrayElemType (ilElemTy.TypeRef.Name), m))
+        else
+            ILAttribElem.Array(ilElemTy, List.map (fun arg -> GenAttribArg amap g eenv arg ilElemTy) args)
 
     // Detect 'typeof<ty>' calls
     | TypeOfExpr g ty, _ -> ILAttribElem.Type(Some(GenType amap x.Range eenv.tyenv ty))

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

@@ -79,8 +79,9 @@ type C() =
         |> shouldSucceed
 
     // https://github.com/dotnet/fsharp/issues/12796
-    // Passing an array of a user-defined type as a custom attribute argument is invalid per ECMA 335.
-    // Previously this caused FS0192 internal error in encodeCustomAttrElemType. Now it gives a proper diagnostic.
+    // Empty arrays of user-defined types in custom attributes should compile successfully.
+    // The element type is substituted with System.Object since no elements need encoding.
+    // Previously this caused FS0192 internal error in encodeCustomAttrElemType.
     [<Fact>]
     let ``Issue 12796 - DefaultValue empty array on record field of array type should not cause internal error`` () =
         FSharp
@@ -94,5 +95,4 @@ type B = { [<DefaultValue([||] : A[])>] BField: A[] }
             """
         |> asLibrary
         |> compile
-        |> shouldFail
-        |> withErrorCode 3885
+        |> shouldSucceed