Fix empty array encoding for all unencodable types and add struct tests

- Broaden isEncodableElemType check to also reject non-enum struct types,
  not just boxed reference types. Previously empty struct arrays compiled
  but produced invalid custom attribute metadata (encoded as enum type),
  causing CustomAttributeFormatException at runtime.
- Add positive test for empty struct array in attribute (compile + ILVerify + run).
- Add negative test for non-empty struct array in attribute (expect error 267).

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

Author: T-Gro

src/Compiler/CodeGen/IlxGen.fs

@@ -10335,19 +10335,32 @@ and GenAttribArg amap (g: TcGlobals) eenv x (ilArgTy: ILType) =
     | Expr.Op(TOp.Array, [ elemTy ], args, m), _ ->
         let ilElemTy = GenType amap m eenv.tyenv elemTy
 
-        // Check if element type can be encoded in custom attribute metadata (ECMA 335).
+        // Check if element type can be encoded in custom attribute metadata (ECMA-335 II.23.3).
         // Valid element types: primitives, enums, string, System.Type, System.Object.
-        let isUnencodableBoxedElemType =
+        let isEncodableElemType =
             match ilElemTy with
-            | ILType.Boxed tspec when
-                tspec.Name <> "System.String"
-                && tspec.Name <> "System.Object"
-                && tspec.Name <> "System.Type"
-                ->
-                true
+            | ILType.Value tspec ->
+                match tspec.Name with
+                | "System.SByte"
+                | "System.Byte"
+                | "System.Int16"
+                | "System.UInt16"
+                | "System.Int32"
+                | "System.UInt32"
+                | "System.Int64"
+                | "System.UInt64"
+                | "System.Double"
+                | "System.Single"
+                | "System.Char"
+                | "System.Boolean" -> true
+                | _ -> isEnumTy g elemTy
+            | ILType.Boxed tspec ->
+                tspec.Name = "System.String"
+                || tspec.Name = "System.Object"
+                || tspec.Name = "System.Type"
             | _ -> false
 
-        if isUnencodableBoxedElemType then
+        if not isEncodableElemType then
             if args.IsEmpty then
                 // Empty arrays: substitute System.Object as element type since no elements need encoding.
                 ILAttribElem.Array(g.ilg.typ_Object, [])

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

@@ -343,3 +343,56 @@ type B() = class end
         |> shouldFail
         |> withErrorCode 3885
         |> withDiagnosticMessageMatches "not a valid custom attribute argument type"
+
+    // https://github.com/dotnet/fsharp/issues/12796
+    [<Fact>]
+    let ``Issue 12796 - Non-empty array of struct type in attribute gives proper error`` () =
+        FSharp
+            """
+module TestModule
+
+open System.ComponentModel
+
+[<Struct>]
+type MyStruct = { X: int }
+
+[<DefaultValue([| { X = 1 } |] : MyStruct[])>]
+type B() = class end
+            """
+        |> compile
+        |> shouldFail
+        |> withErrorCode 267
+
+    // https://github.com/dotnet/fsharp/issues/12796
+    [<Fact>]
+    let ``Issue 12796 - Empty array of struct type in attribute compiles, verifies IL, and runs`` () =
+        FSharp
+            """
+module TestModule
+
+open System
+open System.ComponentModel
+open System.Reflection
+
+[<Struct>]
+type MyStruct = { X: int; Y: int }
+
+[<DefaultValue([||] : MyStruct[])>]
+type T() = class end
+
+[<EntryPoint>]
+let main _ =
+    let attr = typeof<T>.GetCustomAttribute<DefaultValueAttribute>()
+    let arr = attr.Value :?> obj[]
+    if arr.Length <> 0 then failwith "Expected empty array"
+    printfn "struct: len=%d" arr.Length
+    0
+            """
+        |> asExe
+        |> compile
+        |> shouldSucceed
+        |> verifyPEFileWithSystemDlls
+        |> shouldSucceed
+        |> run
+        |> shouldSucceed
+        |> withStdOutContains "struct: len=0"