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

@@ -19,6 +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 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

@@ -10217,6 +10217,18 @@ 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
+        |> verifyCompile
+        |> 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 +1,91 @@
+// #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
+
+open System
+open System.Runtime.InteropServices
+
+type BoolAttribute(name : string, flag : bool) =
+    inherit Attribute()
+    new([<Optional>] flag : bool) = BoolAttribute("", flag)
+
+type IntAttribute(name : string, value : int) =
+    inherit Attribute()
+    new([<Optional>] value : int) = IntAttribute("", value)
+
+type ByteAttribute(name : string, value : byte) =
+    inherit Attribute()
+    new([<Optional>] value : byte) = ByteAttribute("", value)
+
+type SByteAttribute(name : string, value : sbyte) =
+    inherit Attribute()
+    new([<Optional>] value : sbyte) = SByteAttribute("", value)
+
+type Int16Attribute(name : string, value : int16) =
+    inherit Attribute()
+    new([<Optional>] value : int16) = Int16Attribute("", value)
+
+type Int64Attribute(name : string, value : int64) =
+    inherit Attribute()
+    new([<Optional>] value : int64) = Int64Attribute("", value)
+
+type UInt16Attribute(name : string, value : uint16) =
+    inherit Attribute()
+    new([<Optional>] value : uint16) = UInt16Attribute("", value)
+
+type UInt32Attribute(name : string, value : uint32) =
+    inherit Attribute()
+    new([<Optional>] value : uint32) = UInt32Attribute("", value)
+
+type UInt64Attribute(name : string, value : uint64) =
+    inherit Attribute()
+    new([<Optional>] value : uint64) = UInt64Attribute("", value)
+
+type FloatAttribute(name : string, value : float) =
+    inherit Attribute()
+    new([<Optional>] value : float) = FloatAttribute("", value)
+
+type SingleAttribute(name : string, value : float32) =
+    inherit Attribute()
+    new([<Optional>] value : float32) = SingleAttribute("", value)
+
+type CharAttribute(name : string, value : char) =
+    inherit Attribute()
+    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
+