[gcc r15-6047] libstdc++: Fix debug containers for constant evaluation [PR117962]

[Jonathan Wakely]

https://gcc.gnu.org/g:6fbe9e65645f54cc564a928bc0bc69c8b421cb98

commit r15-6047-g6fbe9e65645f54cc564a928bc0bc69c8b421cb98
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Dec 9 10:52:10 2024 +0000

    libstdc++: Fix debug containers for constant evaluation [PR117962]
    
    Using a stateful allocator with std::vector would fail in Debug Mode,
    because the allocator-extended move constructor tries to swap all the
    attached safe iterators, but that uses a non-inline function which isn't
    constexpr. We don't actually need to swap any iterators in constant
    expressions, because we never attach them to the container in the first
    place.
    
    This bug went unnoticed because the tests for constexpr std::vector were
    using a stateful allocator with a std::allocator base class, but were
    failing to override the inherited is_always_equal trait from
    std::allocator. That meant that the allocators took the always-equal
    code paths, and didn't try to use the buggy constructor. In C++26 the
    std::allocator::is_always_equal trait goes away, and so the tests
    changed behaviour, revealing the bug.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/117962
            * include/debug/safe_container.h: Make allocator-extended move
            constructor a no-op during constant evaluation.

Diff:
---
 libstdc++-v3/include/debug/safe_container.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/debug/safe_container.h b/libstdc++-v3/include/debug/safe_container.h
index 9a6c4f7ba743..75c6834bfaec 100644
--- a/libstdc++-v3/include/debug/safe_container.h
+++ b/libstdc++-v3/include/debug/safe_container.h
@@ -64,10 +64,13 @@ namespace __gnu_debug
       _Safe_container(_Safe_container&& __x, const _Alloc& __a, std::false_type)
       : _Safe_container()
       {
-	if (__x._M_cont().get_allocator() == __a)
-	  _Base::_M_swap(__x);
-	else if (!std::__is_constant_evaluated())
-	  __x._M_invalidate_all();
+	if (!std::__is_constant_evaluated())
+	  {
+	    if (__x._M_cont().get_allocator() == __a)
+	      _Base::_M_swap(__x);
+	    else
+	      __x._M_invalidate_all();
+	  }
       }
 
     protected: