[gcc r16-8101] libstdc++: [_GLIBCXX_DEBUG] _M_invalidate_if incompatible with C++20 [PR124444]

[Francois Dumont]

https://gcc.gnu.org/g:a1f1a60fc6490e677d63b637cc268b513749cc5c

commit r16-8101-ga1f1a60fc6490e677d63b637cc268b513749cc5c
Author: François Dumont <frs.dumont@gmail.com>
Date:   Sat Mar 14 14:34:29 2026 +0100

    libstdc++: [_GLIBCXX_DEBUG] _M_invalidate_if incompatible with C++20 [PR124444]
    
    __gnu_cxx::__scoped_lock cannot be instantiated in a C++20 constexpr function. Use
    an intermediate method to avoid this situation.
    
    libstdc++-v3/ChangeLog:
            PR libstdc++/124444
            * include/debug/safe_sequence.h (_Safe_sequence::_M_invalidate_if_impl): New.
            (_Safe_sequence::_M_invalidate_if): Call later if not __is_constant_evaluated.
            * include/debug/safe_sequence.tcc: Rename _M_invalidate_if into
            _M_invalidate_if_impl and remove C++20 constexpr.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/debug/safe_sequence.h   | 12 +++++++++++-
 libstdc++-v3/include/debug/safe_sequence.tcc |  7 ++-----
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/debug/safe_sequence.h b/libstdc++-v3/include/debug/safe_sequence.h
index e7658bb26a77..4415f689c9fe 100644
--- a/libstdc++-v3/include/debug/safe_sequence.h
+++ b/libstdc++-v3/include/debug/safe_sequence.h
@@ -110,6 +110,10 @@ namespace __gnu_debug
   template<typename _Sequence>
     class _Safe_sequence : public _Safe_sequence_base
     {
+      template<typename _Predicate>
+	void
+	_M_invalidate_if_impl(_Predicate __pred) const;
+
     public:
       /** Invalidates all iterators @c x that reference this sequence,
 	  are not singular, and for which @c __pred(x) returns @c
@@ -117,7 +121,13 @@ namespace __gnu_debug
 	  in the safe ones. */
       template<typename _Predicate>
 	_GLIBCXX20_CONSTEXPR void
-	_M_invalidate_if(_Predicate __pred) const;
+	_M_invalidate_if(_Predicate __pred) const
+	{
+	  if (std::__is_constant_evaluated())
+	    return;
+
+	  _M_invalidate_if_impl(__pred);
+	}
 
       /** Transfers all iterators @c x that reference @c from sequence,
 	  are not singular, and for which @c __pred(x) returns @c
diff --git a/libstdc++-v3/include/debug/safe_sequence.tcc b/libstdc++-v3/include/debug/safe_sequence.tcc
index 884478fad9d7..f55767258be5 100644
--- a/libstdc++-v3/include/debug/safe_sequence.tcc
+++ b/libstdc++-v3/include/debug/safe_sequence.tcc
@@ -33,13 +33,10 @@ namespace __gnu_debug
 {
   template<typename _Sequence>
     template<typename _Predicate>
-      _GLIBCXX20_CONSTEXPR void
+      void
       _Safe_sequence<_Sequence>::
-      _M_invalidate_if(_Predicate __pred) const
+      _M_invalidate_if_impl(_Predicate __pred) const
       {
-	if (std::__is_constant_evaluated())
-	  return;
-
 	typedef typename _Sequence::iterator iterator;
 	typedef typename _Sequence::const_iterator const_iterator;