Add support for voption union cases in attribute arguments

This change allows ValueNone and ValueSome to be used as attribute argument values by recognizing union case expressions as valid constant expressions in attributes.

Co-authored-by: T-Gro <[email protected]>

Author: Copilot

src/Compiler/Checking/PostInferenceChecks.fs

@@ -1974,6 +1974,8 @@ and CheckAttribArgExpr cenv env expr =
     | AttribBitwiseOrExpr g (arg1, arg2) ->
         CheckAttribArgExpr cenv env arg1
         CheckAttribArgExpr cenv env arg2
+    | Expr.Op (TOp.UnionCase _, _, args, _) ->
+        List.iter (CheckAttribArgExpr cenv env) args
     | _ ->
         if cenv.reportErrors then
            errorR (Error (FSComp.SR.chkInvalidCustAttrVal(), expr.Range))

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -10329,6 +10329,9 @@ let rec EvalAttribArgExpr suppressLangFeatureCheck (g: TcGlobals) (x: Expr) =
         | _ ->
             errorR (Error ( FSComp.SR.tastNotAConstantExpression(), x.Range))
             x
+    | Expr.Op (TOp.UnionCase _, _, _args, _) ->
+        // Union cases (like ValueNone, Some x) are valid constant expressions for attributes
+        x
     | _ -> 
         errorR (Error ( FSComp.SR.tastNotAConstantExpression(), x.Range))
         x
@@ -10338,6 +10341,8 @@ and EvaledAttribExprEquality g e1 e2 =
     | Expr.Const (c1, _, _), Expr.Const (c2, _, _) -> c1 = c2
     | TypeOfExpr g ty1, TypeOfExpr g ty2 -> typeEquiv g ty1 ty2
     | TypeDefOfExpr g ty1, TypeDefOfExpr g ty2 -> typeEquiv g ty1 ty2
+    | Expr.Op (TOp.UnionCase (UnionCaseRef(tc1, nm1)), _, args1, _), Expr.Op (TOp.UnionCase (UnionCaseRef(tc2, nm2)), _, args2, _) ->
+        tyconRefEq g tc1 tc2 && nm1 = nm2 && (args1.Length = args2.Length) && List.forall2 (EvaledAttribExprEquality g) args1 args2
     | _ -> false
 
 [<return: Struct>]