[gcc r15-11255] libstdc++: Make more _Safe_iterator functions in constexpr in C++20.

[Jonathan Wakely]

https://gcc.gnu.org/g:2bb5a6255821e3588c476541d78a010203db1ee5

commit r15-11255-g2bb5a6255821e3588c476541d78a010203db1ee5
Author: Tomasz Kamiński <tkaminsk@redhat.com>
Date:   Fri Dec 19 13:07:00 2025 +0100

    libstdc++: Make more _Safe_iterator functions in constexpr in C++20.
    
    This functions are indirectly called from flat_ containers operations
    (from preconditions check of lower_bound, upper_bound, ...) that were
    made constexpr by r16-6026-gbf9dd44a97400e, leading to test with
    in _GLIBCXX_DEBUG mode.
    
    For __can_advance we uncoditionally return true in constant evaluation,
    similary to __valid_range. The constexpr iterator will detect comparision
    of iterator to different ranges.
    
    libstdc++-v3/ChangeLog:
    
            * include/debug/helper_functions.h (__gnu_debug::__can_advance):
            Declare as _GLIBCXX20_CONSTEXPR.
            * include/debug/safe_iterator.h (__gnu_debug::__can_advance):
            Define as _GLIBCXX20_CONSTEXPR, and return true for constexpr
            evaluation.
            (__gnu_debug::__base): Define as _GLIBCXX20_CONSTEXPR.
    
    (cherry picked from commit 158acb680582d99d359eb4e609e956aaf1d45ab4)

Diff:
---
 libstdc++-v3/include/debug/helper_functions.h |  4 ++--
 libstdc++-v3/include/debug/safe_iterator.h    | 18 +++++++++++++++---
 2 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/debug/helper_functions.h b/libstdc++-v3/include/debug/helper_functions.h
index b9771034bf34..b058408a1e3f 100644
--- a/libstdc++-v3/include/debug/helper_functions.h
+++ b/libstdc++-v3/include/debug/helper_functions.h
@@ -292,7 +292,7 @@ namespace __gnu_debug
 
   template<typename _Iterator, typename _Sequence, typename _Category,
 	   typename _Size>
-    bool
+    _GLIBCXX20_CONSTEXPR bool
     __can_advance(const _Safe_iterator<_Iterator, _Sequence, _Category>&,
 		  _Size);
 
@@ -304,7 +304,7 @@ namespace __gnu_debug
 
   template<typename _Iterator, typename _Sequence, typename _Category,
 	   typename _Diff>
-    bool
+    _GLIBCXX20_CONSTEXPR bool
     __can_advance(const _Safe_iterator<_Iterator, _Sequence, _Category>&,
 		  const std::pair<_Diff, _Distance_precision>&, int);
 
diff --git a/libstdc++-v3/include/debug/safe_iterator.h b/libstdc++-v3/include/debug/safe_iterator.h
index 9c5f360414bf..0a5ef0097751 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -1139,21 +1139,33 @@ namespace __gnu_debug
 
   template<typename _Iterator, typename _Sequence, typename _Category,
 	   typename _Size>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     __can_advance(const _Safe_iterator<_Iterator, _Sequence, _Category>& __it,
 		  _Size __n)
-    { return __it._M_can_advance(__n); }
+    { 
+      if (std::__is_constant_evaluated())
+	return true;
+
+      return __it._M_can_advance(__n);
+    }
 
   template<typename _Iterator, typename _Sequence, typename _Category,
 	   typename _Diff>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     __can_advance(const _Safe_iterator<_Iterator, _Sequence, _Category>& __it,
 		  const std::pair<_Diff, _Distance_precision>& __dist,
 		  int __way)
-    { return __it._M_can_advance(__dist, __way); }
+    {
+      if (std::__is_constant_evaluated())
+	return true;
+    
+      return __it._M_can_advance(__dist, __way);
+    }
 
   template<typename _Iterator, typename _Sequence>
-    _Iterator
+    _GLIBCXX20_CONSTEXPR _Iterator
     __base(const _Safe_iterator<_Iterator, _Sequence,
 				std::random_access_iterator_tag>& __it)
     { return __it.base(); }