Fix internal error when using custom attribute with optional value type argument (#8353)

GenAttribArg in IlxGen.fs handles Const.Zero (the default for [<Optional>] params
without [<DefaultParameterValue>]) for System.Object, System.String, and System.Type,
but not for primitive value types. This causes an internal error (FS0073) when
applying an attribute with an [<Optional>] value type parameter and no default.

Add Const.Zero -> default value mappings for all 12 primitive types valid in custom
attributes: bool, sbyte, int16, int32, int64, byte, uint16, uint32, uint64, single,
double, char.

Fixes #8353

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

Author: T-Gro

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

@@ -26,6 +26,7 @@
 * Fix box instruction for literal upcasts. (Issue [#18319](https://github.com/dotnet/fsharp/issues/18319), [PR #19338](https://github.com/dotnet/fsharp/pull/19338))
 * Fix Decimal Literal causes InvalidProgramException in Debug builds. (Issue [#18956](https://github.com/dotnet/fsharp/issues/18956), [PR #19338](https://github.com/dotnet/fsharp/pull/19338))
 * 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))
+* Fix internal error when using custom attribute with `[<Optional>]` value type parameter and no `[<DefaultParameterValue>]`. ([Issue #8353](https://github.com/dotnet/fsharp/issues/8353), [PR #19484](https://github.com/dotnet/fsharp/pull/19484))
 
 ### Added
 

src/Compiler/CodeGen/IlxGen.fs

@@ -10328,6 +10328,18 @@ and GenAttribArg amap g eenv x (ilArgTy: ILType) =
         | Const.Zero when isobj -> ILAttribElem.Null
         | Const.Zero when tynm = "System.String" -> ILAttribElem.String None
         | Const.Zero when tynm = "System.Type" -> ILAttribElem.Type None
+        | Const.Zero when tynm = "System.Boolean" -> ILAttribElem.Bool false
+        | Const.Zero when tynm = "System.SByte" -> ILAttribElem.SByte 0y
+        | Const.Zero when tynm = "System.Int16" -> ILAttribElem.Int16 0s
+        | Const.Zero when tynm = "System.Int32" -> ILAttribElem.Int32 0
+        | Const.Zero when tynm = "System.Int64" -> ILAttribElem.Int64 0L
+        | Const.Zero when tynm = "System.Byte" -> ILAttribElem.Byte 0uy
+        | Const.Zero when tynm = "System.UInt16" -> ILAttribElem.UInt16 0us
+        | Const.Zero when tynm = "System.UInt32" -> ILAttribElem.UInt32 0u
+        | Const.Zero when tynm = "System.UInt64" -> ILAttribElem.UInt64 0UL
+        | Const.Zero when tynm = "System.Single" -> ILAttribElem.Single 0.0f
+        | Const.Zero when tynm = "System.Double" -> ILAttribElem.Double 0.0
+        | Const.Zero when tynm = "System.Char" -> ILAttribElem.Char '\000'
         | Const.String i when isobj || tynm = "System.String" -> ILAttribElem.String(Some i)
         | _ -> error (InternalError("The type '" + tynm + "' may not be used as a custom attribute value", m))
 

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/Basic.fs

@@ -303,6 +303,14 @@ module CustomAttributes_Basic =
         |> verifyCompileAndRun
         |> shouldSucceed
 
+    // SOURCE=OptionalAttributeArgs.fs						# OptionalAttributeArgs.fs
+    // Regression test for https://github.com/dotnet/fsharp/issues/8353
+    [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"OptionalAttributeArgs.fs"|])>]
+    let ``OptionalAttributeArgs_fs`` compilation =
+        compilation
+        |> verifyCompileAndRun
+        |> shouldSucceed
+
     // SOURCE=W_ReturnType03b.fs          SCFLAGS="--test:ErrorRanges"		# W_ReturnType03b.fs
     [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"W_ReturnType03b.fs"|])>]
     let ``W_ReturnType03b_fs`` compilation =

tests/FSharp.Compiler.ComponentTests/Conformance/BasicGrammarElements/CustomAttributes/Basic/OptionalAttributeArgs.fs

@@ -0,0 +1,127 @@
+// #Regression #Conformance #DeclarationElements #Attributes
+// Regression test for https://github.com/dotnet/fsharp/issues/8353
+// Verify that custom attributes with [<Optional>] parameters (no DefaultParameterValue) compile for all value types
+// and that the emitted default values are correct at runtime.
+
+open System
+open System.Runtime.InteropServices
+
+type BoolAttribute(name: string, flag: bool) =
+    inherit Attribute()
+    member _.Flag = flag
+    new([<Optional>] flag: bool) = BoolAttribute("", flag)
+
+type IntAttribute(name: string, value: int) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: int) = IntAttribute("", value)
+
+type ByteAttribute(name: string, value: byte) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: byte) = ByteAttribute("", value)
+
+type SByteAttribute(name: string, value: sbyte) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: sbyte) = SByteAttribute("", value)
+
+type Int16Attribute(name: string, value: int16) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: int16) = Int16Attribute("", value)
+
+type Int64Attribute(name: string, value: int64) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: int64) = Int64Attribute("", value)
+
+type UInt16Attribute(name: string, value: uint16) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: uint16) = UInt16Attribute("", value)
+
+type UInt32Attribute(name: string, value: uint32) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: uint32) = UInt32Attribute("", value)
+
+type UInt64Attribute(name: string, value: uint64) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: uint64) = UInt64Attribute("", value)
+
+type FloatAttribute(name: string, value: float) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: float) = FloatAttribute("", value)
+
+type SingleAttribute(name: string, value: float32) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: float32) = SingleAttribute("", value)
+
+type CharAttribute(name: string, value: char) =
+    inherit Attribute()
+    member _.Value = value
+    new([<Optional>] value: char) = CharAttribute("", value)
+
+[<Bool>]
+type T1() = class end
+
+[<Int>]
+type T2() = class end
+
+[<Byte>]
+type T3() = class end
+
+[<Float>]
+type T4() = class end
+
+[<Single>]
+type T5() = class end
+
+[<Char>]
+type T6() = class end
+
+[<SByte>]
+type T7() = class end
+
+[<Int16>]
+type T8() = class end
+
+[<Int64>]
+type T9() = class end
+
+[<UInt16>]
+type T10() = class end
+
+[<UInt32>]
+type T11() = class end
+
+[<UInt64>]
+type T12() = class end
+
+// Verify default values at runtime via reflection
+let inline getAttr<'a when 'a :> Attribute> (t: Type) = t.GetCustomAttributes(typeof<'a>, false).[0] :?> 'a
+
+let check (name: string) (actual: 'a) (expected: 'a) =
+    if actual <> expected then
+        failwithf "%s: expected %A but got %A" name expected actual
+
+[<EntryPoint>]
+let main _ =
+    check "bool" (getAttr<BoolAttribute>(typeof<T1>)).Flag false
+    check "int" (getAttr<IntAttribute>(typeof<T2>)).Value 0
+    check "byte" (getAttr<ByteAttribute>(typeof<T3>)).Value 0uy
+    check "float" (getAttr<FloatAttribute>(typeof<T4>)).Value 0.0
+    check "single" (getAttr<SingleAttribute>(typeof<T5>)).Value 0.0f
+    check "char" (getAttr<CharAttribute>(typeof<T6>)).Value '\000'
+    check "sbyte" (getAttr<SByteAttribute>(typeof<T7>)).Value 0y
+    check "int16" (getAttr<Int16Attribute>(typeof<T8>)).Value 0s
+    check "int64" (getAttr<Int64Attribute>(typeof<T9>)).Value 0L
+    check "uint16" (getAttr<UInt16Attribute>(typeof<T10>)).Value 0us
+    check "uint32" (getAttr<UInt32Attribute>(typeof<T11>)).Value 0u
+    check "uint64" (getAttr<UInt64Attribute>(typeof<T12>)).Value 0UL
+    0
+