Improve #12796 tests: add class type test, clean up test names and assertions

Agent-Logs-Url: https://github.com/dotnet/fsharp/sessions/bbf54c23-63b8-4d19-b957-5a08ae2c20c3

Co-authored-by: T-Gro <46543583+T-Gro@users.noreply.github.com>

Author: Copilot

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

@@ -191,33 +191,44 @@ type C() = class end
         |> shouldSucceed
 
     // https://github.com/dotnet/fsharp/issues/12796
-    // Non-empty arrays of user-defined types in custom attributes should produce FS3885 diagnostic.
+    // Empty arrays of user-defined class types in custom attributes should compile successfully.
     // Previously this caused FS0192 internal error in encodeCustomAttrElemType.
     [<Fact>]
-    let ``Issue 12796 - Non-empty array of user-defined type in attribute gives proper error`` () =
+    let ``Issue 12796 - Empty array of class type in attribute compiles, verifies IL, and runs`` () =
         FSharp
             """
 module TestModule
 
+open System
 open System.ComponentModel
+open System.Reflection
 
 [<AllowNullLiteral>]
 type A() = class end
 
-[<DefaultValue([| (null : A) |])>]
+[<DefaultValue([||] : A[])>]
 type B() = class end
+
+[<EntryPoint>]
+let main _ =
+    let attr = typeof<B>.GetCustomAttribute<DefaultValueAttribute>()
+    let arr = attr.Value :?> obj[]
+    if arr.Length <> 0 then failwith "Expected empty array"
+    printfn "class: len=%d" arr.Length
+    0
             """
+        |> asExe
         |> compile
-        |> shouldFail
-        |> withErrorCode 3885
-        |> withDiagnosticMessageMatches "not a valid custom attribute argument type"
+        |> shouldSucceed
+        |> verifyPEFileWithSystemDlls
+        |> shouldSucceed
+        |> run
+        |> shouldSucceed
+        |> withStdOutContains "class: len=0"
 
     // https://github.com/dotnet/fsharp/issues/12796
-    // 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 - Empty array of user-defined type in attribute compiles runs and verifies IL`` () =
+    let ``Issue 12796 - Empty array of record type in attribute compiles, verifies IL, and runs`` () =
         FSharp
             """
 module TestModule
@@ -235,7 +246,8 @@ type B() = class end
 let main _ =
     let attr = typeof<B>.GetCustomAttribute<DefaultValueAttribute>()
     let arr = attr.Value :?> obj[]
-    printfn "len=%d" arr.Length
+    if arr.Length <> 0 then failwith "Expected empty array"
+    printfn "record: len=%d" arr.Length
     0
             """
         |> asExe
@@ -245,11 +257,11 @@ let main _ =
         |> shouldSucceed
         |> run
         |> shouldSucceed
-        |> withStdOutContains "len=0"
+        |> withStdOutContains "record: len=0"
 
     // https://github.com/dotnet/fsharp/issues/12796
     [<Fact>]
-    let ``Issue 12796 - Empty array of enum type in attribute compiles runs and verifies IL`` () =
+    let ``Issue 12796 - Empty array of enum type in attribute compiles, verifies IL, and runs`` () =
         FSharp
             """
 module TestModule
@@ -267,7 +279,8 @@ type T() = class end
 let main _ =
     let attr = typeof<T>.GetCustomAttribute<DefaultValueAttribute>()
     let arr = attr.Value :?> MyEnum[]
-    printfn "len=%d" arr.Length
+    if arr.Length <> 0 then failwith "Expected empty array"
+    printfn "enum: len=%d" arr.Length
     0
             """
         |> asExe
@@ -277,11 +290,11 @@ let main _ =
         |> shouldSucceed
         |> run
         |> shouldSucceed
-        |> withStdOutContains "len=0"
+        |> withStdOutContains "enum: len=0"
 
     // https://github.com/dotnet/fsharp/issues/12796
     [<Fact>]
-    let ``Issue 12796 - Empty array of primitive type in attribute compiles runs and verifies IL`` () =
+    let ``Issue 12796 - Empty array of primitive type in attribute compiles, verifies IL, and runs`` () =
         FSharp
             """
 module TestModule
@@ -297,7 +310,8 @@ type T() = class end
 let main _ =
     let attr = typeof<T>.GetCustomAttribute<DefaultValueAttribute>()
     let arr = attr.Value :?> int[]
-    printfn "len=%d" arr.Length
+    if arr.Length <> 0 then failwith "Expected empty array"
+    printfn "int: len=%d" arr.Length
     0
             """
         |> asExe
@@ -307,4 +321,25 @@ let main _ =
         |> shouldSucceed
         |> run
         |> shouldSucceed
-        |> withStdOutContains "len=0"
+        |> withStdOutContains "int: len=0"
+
+    // https://github.com/dotnet/fsharp/issues/12796
+    // Non-empty arrays of user-defined types should produce FS3885 diagnostic instead of FS0192 internal error.
+    [<Fact>]
+    let ``Issue 12796 - Non-empty array of user-defined type in attribute gives proper error`` () =
+        FSharp
+            """
+module TestModule
+
+open System.ComponentModel
+
+[<AllowNullLiteral>]
+type A() = class end
+
+[<DefaultValue([| (null : A) |])>]
+type B() = class end
+            """
+        |> compile
+        |> shouldFail
+        |> withErrorCode 3885
+        |> withDiagnosticMessageMatches "not a valid custom attribute argument type"