[committed] libstdc++: Make more _Safe_iterator functions in constexpr in C++20.

[Tomasz Kamiński]

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.
---
Tested on x86_64-linux. Pushed to trunk.

 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 b9771034bf3..b058408a1e3 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 80ed9570ecb..ef61ab5e026 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -1145,21 +1145,33 @@ 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(); }
-- 
2.52.0