From 260a884ee62df7fd3b6b661baf64b99eb516ef29 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Thu, 4 Jul 2024 16:30:53 +0800
Subject: [PATCH] Backport overflow checking of `lcm`/`gcd` to C++17

`_Mul_overflow` is made to only handle non-`bool` integer types in
C++17.
---
 stl/inc/numeric  | 11 +++--------
 stl/inc/xutility | 14 +++++++++++---
 2 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/stl/inc/numeric b/stl/inc/numeric
index ce5e131b11..0dafdfa9e3 100644
--- a/stl/inc/numeric
+++ b/stl/inc/numeric
@@ -625,10 +625,9 @@ _NODISCARD constexpr common_type_t<_Mt, _Nt> gcd(const _Mt _Mx, const _Nt _Nx) n
     using _Common          = common_type_t<_Mt, _Nt>;
     using _Common_unsigned = make_unsigned_t<_Common>;
 
-#if _HAS_CXX20
     if constexpr (is_signed_v<_Common>) {
 #ifndef _DEBUG
-        if (_STD is_constant_evaluated())
+        if (_STD _Is_constant_evaluated())
 #endif // ^^^ !defined(_DEBUG) ^^
         {
             if (_Mx == _STD _Min_limit<_Common>() || _Nx == _STD _Min_limit<_Common>()) {
@@ -637,7 +636,6 @@ _NODISCARD constexpr common_type_t<_Mt, _Nt> gcd(const _Mt _Mx, const _Nt _Nx) n
             }
         }
     }
-#endif // _HAS_CXX20
 
     return _Select_countr_zero_impl<_Common_unsigned>([=](auto _Countr_zero_impl) {
         _Common_unsigned _Mx_magnitude = _Abs_u(_Mx);
@@ -684,11 +682,10 @@ _NODISCARD constexpr common_type_t<_Mt, _Nt> lcm(const _Mt _Mx, const _Nt _Nx) n
         return 0;
     }
 
-#if _HAS_CXX20
 #ifdef _DEBUG
     if constexpr (true)
 #else // ^^^ defined(_DEBUG) / !defined(_DEBUG) vvv
-    if (_STD is_constant_evaluated())
+    if (_STD _Is_constant_evaluated())
 #endif // ^^^ !defined(_DEBUG) ^^^
     {
         _Common_unsigned _Result = 0;
@@ -700,9 +697,7 @@ _NODISCARD constexpr common_type_t<_Mt, _Nt> lcm(const _Mt _Mx, const _Nt _Nx) n
         }
 
         return static_cast<_Common>(_Result);
-    } else
-#endif // _HAS_CXX20
-    {
+    } else {
         return static_cast<_Common>((_Mx_magnitude / _STD gcd(_Mx_magnitude, _Nx_magnitude)) * _Nx_magnitude);
     }
 }
diff --git a/stl/inc/xutility b/stl/inc/xutility
index eb644e877f..2beb7c6dd2 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -7374,16 +7374,23 @@ _NODISCARD constexpr bool _Add_overflow(const _Int _Left, const _Int _Right, _In
 }
 #endif // _HAS_CXX23
 
+#if _HAS_CXX17
 #if _HAS_CXX20
 template <_Integer_like _Int>
+#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
+template <class _Int, enable_if_t<_Is_nonbool_integral<_Int>, int> = 0>
+#endif // ^^^ !_HAS_CXX20 ^^^
 _NODISCARD constexpr bool _Mul_overflow(const _Int _Left, const _Int _Right, _Int& _Out) {
+#if defined(__clang__) && !_HAS_CXX20
+    return __builtin_mul_overflow(_Left, _Right, &_Out);
+#else // ^^^ defined(__clang__) && !_HAS_CXX20 / !defined(__clang__) || _HAS_CXX20 vvv
 #ifdef __clang__
-    if constexpr (integral<_Int>) {
+    if constexpr (is_integral_v<_Int>) {
         return __builtin_mul_overflow(_Left, _Right, &_Out);
     } else
 #endif // defined(__clang__)
     {
-        if constexpr (!_Signed_integer_like<_Int>) {
+        if constexpr (static_cast<_Int>(-1) > static_cast<_Int>(0)) {
             constexpr auto _UInt_max = _STD _Max_limit<_Int>();
             const bool _Overflow     = _Left != 0 && _Right > _UInt_max / _Left;
             if (!_Overflow) {
@@ -7422,8 +7429,9 @@ _NODISCARD constexpr bool _Mul_overflow(const _Int _Left, const _Int _Right, _In
             // ^^^ Based on llvm::MulOverflow ^^^
         }
     }
+#endif // ^^^ !defined(__clang__) || _HAS_CXX20 ^^^
 }
-#endif // _HAS_CXX20
+#endif // _HAS_CXX17
 
 _STD_END