[gcc r15-11254] libstdc++: Backport constexpr support for debug mode helpers [PR119163]

[Jonathan Wakely]

https://gcc.gnu.org/g:6e0376b42d4c6eff348a151b0f66453c5a5d8f00

commit r15-11254-g6e0376b42d4c6eff348a151b0f66453c5a5d8f00
Author: François Dumont <frs.dumont@gmail.com>
Date:   Mon Sep 22 18:58:52 2025 +0200

    libstdc++: Backport constexpr support for debug mode helpers [PR119163]
    
    This is a partial backport of r16-5845-g8a2e6590cc4a2f to fix a test
    failure with -D_GLIBCXX_DEBUG:
    
    FAIL: 20_util/specialized_algorithms/uninitialized_copy/constexpr.cc  -std=gnu++26 (test for excess errors)
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/119163
            * include/debug/functions.h (__check_valid_range): Add C++20 constexpr.
            * include/debug/helper_functions.h (__valid_range): Likewise.
            * include/debug/safe_iterator.h (__valid_range): Likewise.

Diff:
---
 libstdc++-v3/include/debug/functions.h        | 10 +++++++---
 libstdc++-v3/include/debug/helper_functions.h |  4 ++--
 libstdc++-v3/include/debug/safe_iterator.h    | 12 +++++++++++-
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/debug/functions.h b/libstdc++-v3/include/debug/functions.h
index c5f5582b0050..4e6fdae71935 100644
--- a/libstdc++-v3/include/debug/functions.h
+++ b/libstdc++-v3/include/debug/functions.h
@@ -53,15 +53,19 @@ namespace __gnu_debug
    * assertion statement because, e.g., we are in a constructor.
   */
   template<typename _InputIterator>
-    inline _InputIterator
+    _GLIBCXX20_CONSTEXPR inline _InputIterator
     __check_valid_range(const _InputIterator& __first,
 			const _InputIterator& __last,
 			const char* __file,
 			unsigned int __line,
 			const char* __function)
     {
-      __glibcxx_check_valid_range_at(__first, __last,
-				     __file, __line, __function);
+      if (!std::__is_constant_evaluated())
+	{
+	  __glibcxx_check_valid_range_at(__first, __last,
+					 __file, __line, __function);
+	}
+
       return __first;
     }
 
diff --git a/libstdc++-v3/include/debug/helper_functions.h b/libstdc++-v3/include/debug/helper_functions.h
index 9395892b537c..b9771034bf34 100644
--- a/libstdc++-v3/include/debug/helper_functions.h
+++ b/libstdc++-v3/include/debug/helper_functions.h
@@ -249,7 +249,7 @@ namespace __gnu_debug
     }
 
   template<typename _Iterator, typename _Sequence, typename _Category>
-    bool
+    _GLIBCXX20_CONSTEXPR bool
     __valid_range(const _Safe_iterator<_Iterator, _Sequence, _Category>&,
 		  const _Safe_iterator<_Iterator, _Sequence, _Category>&,
 		  typename _Distance_traits<_Iterator>::__type&);
@@ -272,7 +272,7 @@ namespace __gnu_debug
     }
 
   template<typename _Iterator, typename _Sequence, typename _Category>
-    bool
+    _GLIBCXX20_CONSTEXPR bool
     __valid_range(const _Safe_iterator<_Iterator, _Sequence, _Category>&,
 		  const _Safe_iterator<_Iterator, _Sequence, _Category>&);
 
diff --git a/libstdc++-v3/include/debug/safe_iterator.h b/libstdc++-v3/include/debug/safe_iterator.h
index 7c563381d0bf..9c5f360414bf 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -1108,21 +1108,31 @@ namespace __gnu_debug
 
   /** Safe iterators know how to check if they form a valid range. */
   template<typename _Iterator, typename _Sequence, typename _Category>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     __valid_range(const _Safe_iterator<_Iterator, _Sequence,
 				       _Category>& __first,
 		  const _Safe_iterator<_Iterator, _Sequence,
 				       _Category>& __last,
 		  typename _Distance_traits<_Iterator>::__type& __dist)
-    { return __first._M_valid_range(__last, __dist); }
+    {
+      if (std::__is_constant_evaluated())
+	return true;
+
+      return __first._M_valid_range(__last, __dist);
+    }
 
   template<typename _Iterator, typename _Sequence, typename _Category>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     __valid_range(const _Safe_iterator<_Iterator, _Sequence,
 				       _Category>& __first,
 		  const _Safe_iterator<_Iterator, _Sequence,
 				       _Category>& __last)
     {
+      if (std::__is_constant_evaluated())
+	return true;
+
       typename _Distance_traits<_Iterator>::__type __dist;
       return __first._M_valid_range(__last, __dist);
     }