[libcxx-commits] [libcxx] [libc++] P3029R1: Better `mdspan`'s CTAD (PR #87873)

[Mark de Wever via libcxx-commits]

================
@@ -174,6 +196,23 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+#if _LIBCPP_STD_VER >= 26
+
+template <class _Tp>
+concept __integral_constant_like =
+    is_integral_v<decltype(_Tp::value)> && !is_same_v<bool, remove_const_t<decltype(_Tp::value)>> &&
+    convertible_to<_Tp, decltype(_Tp::value)> && equality_comparable_with<_Tp, decltype(_Tp::value)> &&
+    bool_constant<_Tp() == _Tp::value>::value &&
+    bool_constant<static_cast<decltype(_Tp::value)>(_Tp()) == _Tp::value>::value;
+
+template <class _Tp>
+constexpr size_t __maybe_static_ext = dynamic_extent;
+
+template <__integral_constant_like _Tp>
+constexpr size_t __maybe_static_ext<_Tp> = {_Tp::value};
+
+#endif // _LIBCPP_STD_VER >= 26
+
----------------
mordante wrote:

Since this is used only once, please move it to where it's used.

https://github.com/llvm/llvm-project/pull/87873