[Bug libstdc++/124444] debug _M_invalidate_if incompatible with C++20 since r16-5845

[François Dumont]

Patch extracted from this PR:

https://forge.sourceware.org/gcc/gcc-TEST/pulls/136
I was able to reproduce the bug in the context  the same PR on those tests:

23_containers/vector/bool/capacity/constexpr.cc
23_containers/vector/bool/modifiers/constexpr.cc
23_containers/vector/modifiers/constexpr.cc

It was the reason of doing this change in the PR.

    libstdc++: [_GLIBCXX_DEBUG] _M_invalidate_if incompatible with 
C++20 [PR124444]

    __gnu_cxx::__scoped_lock cannot be instantiated in a C++20 
constexpr function.

    libstdc++-v3/ChangeLog:

            * 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.

Tested under Linux x86_64 _GLIBCXX_DEBUG mode.

Ok to commit ?

François
diff --git a/libstdc++-v3/include/debug/safe_sequence.h 
b/libstdc++-v3/include/debug/safe_sequence.h
index e7658bb26a7..4415f689c9f 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 884478fad9d..f55767258be 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;