Replace enable_if_t in _Debug_lt_pred with if constexpr and var…

…iable template (#4610)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/xutility

@@ -1513,26 +1513,30 @@ using _Unwrap_enum_t = typename _Unwrap_enum<_Elem>::type;
 #define _DEBUG_ORDER_SET_UNWRAPPED(otherIter, first, last, pred) \
     _STD _Debug_order_set_unchecked<otherIter>(first, last, pred)
 
-template <class _Pr, class _Ty1, class _Ty2,
-    enable_if_t<is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>, int> = 0>
-constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(
-    noexcept(_Pred(_Left, _Right)) && noexcept(_Pred(_Right, _Left))) {
-    // test if _Pred(_Left, _Right) and _Pred is strict weak ordering, when the arguments are the cv-same-type
+template <class _Pr, class _Ty1, class _Ty2>
+constexpr bool _Enable_debug_lt_pred_order_check = is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>;
+
+template <class _Pr, class _Ty1, class _Ty2, bool _Order_check = _Enable_debug_lt_pred_order_check<_Pr, _Ty1, _Ty2>>
+constexpr bool _Debug_lt_pred_order_check_noexcept =
+    noexcept(_STD declval<_Pr&>()(_STD declval<_Ty2&>(), _STD declval<_Ty1&>()));
+
+template <class _Pr, class _Ty1, class _Ty2>
+constexpr bool _Debug_lt_pred_order_check_noexcept<_Pr, _Ty1, _Ty2, false> = true;
+
+template <class _Pr, class _Ty1, class _Ty2>
+_NODISCARD constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(
+    noexcept(_Pred(_Left, _Right)) && _Debug_lt_pred_order_check_noexcept<_Pr, _Ty1, _Ty2>) {
     const auto _Result = static_cast<bool>(_Pred(_Left, _Right));
-    if (_Result) {
-        _STL_VERIFY(!_Pred(_Right, _Left), "invalid comparator");
+
+    if constexpr (_Enable_debug_lt_pred_order_check<_Pr, _Ty1, _Ty2>) {
+        if (_Result) {
+            _STL_VERIFY(!_Pred(_Right, _Left), "invalid comparator");
+        }
     }
 
     return _Result;
 }
 
-template <class _Pr, class _Ty1, class _Ty2,
-    enable_if_t<!is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>, int> = 0>
-constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(noexcept(_Pred(_Left, _Right))) {
-    // test if _Pred(_Left, _Right); no debug checks as the types differ
-    return static_cast<bool>(_Pred(_Left, _Right));
-}
-
 template <class _InIt, class _Sentinel, class _Pr>
 constexpr void _Debug_order_unchecked(_InIt _First, _Sentinel _Last, _Pr&& _Pred) {
     // test if range is ordered by predicate