From 0ed25707efdfaf5646aff315f80dd8fd5f4afeef Mon Sep 17 00:00:00 2001
From: Stephan Lachnit <stephanlachnit@debian.org>
Date: Mon, 18 Nov 2024 13:23:50 +0100
Subject: [PATCH] Add _MDSPAN_USE_IF_CONSTEXPR_17 macro

---
 include/experimental/__p0009_bits/config.hpp | 7 +++++++
 include/experimental/__p0009_bits/macros.hpp | 6 ++++++
 include/experimental/__p0009_bits/mdspan.hpp | 5 +++--
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/include/experimental/__p0009_bits/config.hpp b/include/experimental/__p0009_bits/config.hpp
index e8cacf40..ce2e6f20 100644
--- a/include/experimental/__p0009_bits/config.hpp
+++ b/include/experimental/__p0009_bits/config.hpp
@@ -176,6 +176,13 @@ static_assert(_MDSPAN_CPLUSPLUS >= MDSPAN_CXX_STD_14, "mdspan requires C++14 or
 #  endif
 #endif
 
+#ifndef _MDSPAN_USE_IF_CONSTEXPR_17
+#  if (defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606) \
+        || (!defined(__cpp_constexpr) && MDSPAN_HAS_CXX_17)
+#    define _MDSPAN_USE_IF_CONSTEXPR_17 1
+#  endif
+#endif
+
 #ifndef _MDSPAN_USE_INTEGER_SEQUENCE
 #  if defined(_MDSPAN_COMPILER_MSVC)
 #    if (defined(__cpp_lib_integer_sequence) && __cpp_lib_integer_sequence >= 201304)
diff --git a/include/experimental/__p0009_bits/macros.hpp b/include/experimental/__p0009_bits/macros.hpp
index b60c4261..91c7817e 100644
--- a/include/experimental/__p0009_bits/macros.hpp
+++ b/include/experimental/__p0009_bits/macros.hpp
@@ -697,3 +697,9 @@ struct __bools;
 
 // </editor-fold> end Pre-C++14 constexpr }}}1
 //==============================================================================
+
+#if _MDSPAN_USE_IF_CONSTEXPR_17
+#  define _MDSPAN_IF_CONSTEXPR_17 constexpr
+#else
+#  define _MDSPAN_IF_CONSTEXPR_17
+#endif
diff --git a/include/experimental/__p0009_bits/mdspan.hpp b/include/experimental/__p0009_bits/mdspan.hpp
index 2100492a..9acd4b0f 100644
--- a/include/experimental/__p0009_bits/mdspan.hpp
+++ b/include/experimental/__p0009_bits/mdspan.hpp
@@ -18,6 +18,7 @@
 
 #include "default_accessor.hpp"
 #include "layout_right.hpp"
+#include "macros.hpp"
 #include "extents.hpp"
 #include "trait_backports.hpp"
 #include "compressed_pair.hpp"
@@ -442,7 +443,7 @@ class mdspan
   MDSPAN_FORCE_INLINE_FUNCTION constexpr mapping_type const& __mapping_ref() const noexcept { return __members.__second().__first(); }
   MDSPAN_FORCE_INLINE_FUNCTION _MDSPAN_CONSTEXPR_14 accessor_type& __accessor_ref() noexcept { return __members.__second().__second(); }
   MDSPAN_FORCE_INLINE_FUNCTION constexpr accessor_type const& __accessor_ref() const noexcept { return __members.__second().__second(); }
-  
+
   MDSPAN_TEMPLATE_REQUIRES(
     class SizeType,
     /* requires */ (
@@ -452,7 +453,7 @@ class mdspan
   )
   MDSPAN_FORCE_INLINE_FUNCTION constexpr bool __is_index_oor(SizeType index, index_type extent) const noexcept {
     // Check for negative indices
-    if constexpr (std::is_signed_v<index_type>) {
+    if _MDSPAN_IF_CONSTEXPR_17 (_MDSPAN_TRAIT(std::is_signed, index_type)) {
       if (static_cast<index_type>(index) < 0) {
         return true;
       }