Improve #12796 tests: TDD with compileAndRun, ILVerify, and runtime reflection

- Rewrite tests to use compileExeAndRun pipeline: compile, shouldSucceed,
  verifyPEFileWithSystemDlls, run, verify stdout
- Each test compiles an exe that reads the attribute value via reflection
  at runtime and verifies the array length is 0
- Add ILVerify step (verifyPEFileWithSystemDlls) to validate IL metadata
- Fix decodeCustomAttrElemType to handle 0x51 (SERIALIZATION_TYPE_TAGGED_OBJECT)
  for object[] array element types in custom attribute blobs (ECMA-335 II.23.3)
- Add open FSharp.Test for ILVerifierModule access

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

Author: T-Gro

src/Compiler/AbstractIL/il.fs

@@ -4978,6 +4978,7 @@ let rec decodeCustomAttrElemType bytes sigptr x =
         let elemTy, sigptr = decodeCustomAttrElemType bytes sigptr et
         mkILArr1DTy elemTy, sigptr
     | x when x = 0x50uy -> PrimaryAssemblyILGlobals.typ_Type, sigptr
+    | x when x = 0x51uy -> PrimaryAssemblyILGlobals.typ_Object, sigptr // SERIALIZATION_TYPE_TAGGED_OBJECT (ECMA-335 II.23.3)
     | _ -> failwithf "decodeCustomAttrElemType ilg: unrecognized custom element type: %A" x
 
 /// Given a custom attribute element, encode it to a binary representation according to the rules in Ecma 335 Partition II.

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

@@ -3,6 +3,7 @@
 namespace Language
 
 open Xunit
+open FSharp.Test
 open FSharp.Test.Compiler
 
 module AttributeCheckingTests =
@@ -194,47 +195,94 @@ type C() = class end
     // 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`` () =
+    let ``Issue 12796 - Empty array of user-defined type in attribute compiles runs and verifies IL`` () =
         FSharp
             """
 module TestModule
 
+open System
 open System.ComponentModel
+open System.Reflection
 
 type A = { AField: string }
-type B = { [<DefaultValue([||] : A[])>] BField: A[] }
+
+[<DefaultValue([||] : A[])>]
+type B() = class end
+
+[<EntryPoint>]
+let main _ =
+    let attr = typeof<B>.GetCustomAttribute<DefaultValueAttribute>()
+    let arr = attr.Value :?> obj[]
+    printfn "len=%d" arr.Length
+    0
             """
-        |> asLibrary
+        |> asExe
         |> compile
         |> shouldSucceed
+        |> verifyPEFileWithSystemDlls
+        |> shouldSucceed
+        |> run
+        |> shouldSucceed
+        |> withStdOutContains "len=0"
 
     // https://github.com/dotnet/fsharp/issues/12796
     [<Fact>]
-    let ``Issue 12796 - Empty array of enum type in attribute should succeed`` () =
+    let ``Issue 12796 - Empty array of enum type in attribute compiles runs and verifies IL`` () =
         FSharp
             """
 module TestModule
 
+open System
+open System.ComponentModel
+open System.Reflection
+
 type MyEnum = A = 0 | B = 1
-type MyAttr(v: obj) = inherit System.Attribute()
-type T = { [<MyAttr([||] : MyEnum[])>] F: int }
+
+[<DefaultValue([||] : MyEnum[])>]
+type T() = class end
+
+[<EntryPoint>]
+let main _ =
+    let attr = typeof<T>.GetCustomAttribute<DefaultValueAttribute>()
+    let arr = attr.Value :?> MyEnum[]
+    printfn "len=%d" arr.Length
+    0
             """
-        |> asLibrary
+        |> asExe
         |> compile
         |> shouldSucceed
+        |> verifyPEFileWithSystemDlls
+        |> shouldSucceed
+        |> run
+        |> shouldSucceed
+        |> withStdOutContains "len=0"
 
     // https://github.com/dotnet/fsharp/issues/12796
     [<Fact>]
-    let ``Issue 12796 - Empty array of primitive type in attribute should succeed`` () =
+    let ``Issue 12796 - Empty array of primitive type in attribute compiles runs and verifies IL`` () =
         FSharp
             """
 module TestModule
 
-type MyAttr(v: int[]) = inherit System.Attribute()
-type T = { [<MyAttr([||])>] F: int }
+open System
+open System.ComponentModel
+open System.Reflection
+
+[<DefaultValue([||] : int[])>]
+type T() = class end
+
+[<EntryPoint>]
+let main _ =
+    let attr = typeof<T>.GetCustomAttribute<DefaultValueAttribute>()
+    let arr = attr.Value :?> int[]
+    printfn "len=%d" arr.Length
+    0
             """
-        |> asLibrary
+        |> asExe
         |> compile
         |> shouldSucceed
-
-
+        |> verifyPEFileWithSystemDlls
+        |> shouldSucceed
+        |> run
+        |> shouldSucceed
+        |> withStdOutContains "len=0"