[committed][_GLIBCXX_DEBUG] Simplify constant-evaluated checks

[François Dumont]

Hi

On a previous patch Jonathan spotted that in _GLIBCXX_DEBUG 
implementation of inplace_vector we could use the builtin constant 
evaluated checks rather than the library equivalent.

So here is this simplification outside any patch.

    libstdc++: [_GLIBCXX_DEBUG] Simplify constant-evaluated checks

    In the C++26 std::__debug::inplace_vector implementation we can use:

    if consteval
    {
      ...
    }

    rather than:

    if (std::is_constant_evaluated())

    libstdc++-v3/ChangeLog:

            * include/debug/inplace_vector
            (inplace_vector::emplace): Replace usages of 
std::is_constant_evaluated
            by consteval.
            (inplace_vector::insert): Likewise.
            (inplace_vector::erase): Likewise.

Tested under Linux x86_64 _GLIBCXX_DEBUG mode.

2 tests are already failing on master, I'm working on this independently 
of this patch.

François

diff --git a/libstdc++-v3/include/debug/inplace_vector 
b/libstdc++-v3/include/debug/inplace_vector
index e579e8636f9..a0bf4c85442 100644
--- a/libstdc++-v3/include/debug/inplace_vector
+++ b/libstdc++-v3/include/debug/inplace_vector
@@ -400,10 +400,12 @@ namespace __debug
        constexpr iterator
        emplace(const_iterator __position, _Args&&... __args)
        {
-         if (std::is_constant_evaluated())
+         if consteval
+           {
              return iterator(_Base::emplace(__position.base(),
                                             std::forward<_Args>(__args)...),
                              this);
+           }
 
          __glibcxx_check_insert(__position);
          const difference_type __offset = __position.base() - _Base::cbegin();
@@ -424,8 +426,10 @@ namespace __debug
       constexpr iterator
       insert(const_iterator __position, size_type __n, const _Tp& __x)
       {
-       if (std::is_constant_evaluated())
+       if consteval
+         {
            return iterator(_Base::insert(__position.base(), __n, __x), this);
+         }
 
        __glibcxx_check_insert(__position);
        const difference_type __offset = __position.base() - _Base::cbegin();
@@ -439,10 +443,13 @@ namespace __debug
        insert(const_iterator __position, _InputIterator __first,
               _InputIterator __last)
        {
-         if (std::is_constant_evaluated())
+         if consteval
+           {
              return iterator(_Base::insert(__position.base(),
                                            __gnu_debug::__unsafe(__first),
-                                         __gnu_debug::__unsafe(__last)), this);
+                                           __gnu_debug::__unsafe(__last)),
+                             this);
+           }
 
          typename __gnu_debug::_Distance_traits<_InputIterator>::__type __dist;
          __glibcxx_check_insert_range(__position, __first, __last, __dist);
@@ -476,8 +483,10 @@ namespace __debug
       constexpr iterator
       insert(const_iterator __position, initializer_list<_Tp> __il)
       {
-       if (std::is_constant_evaluated())
+       if consteval
+         {
            return iterator(_Base::insert(__position.base(), __il), this);
+         }
 
        __glibcxx_check_insert(__position);
        const auto __size = size();
@@ -491,8 +500,10 @@ namespace __debug
       constexpr iterator
       erase(const_iterator __position)
       {
-       if (std::is_constant_evaluated())
+       if consteval
+         {
            return iterator(_Base::erase(__position.base()), this);
+         }
 
        __glibcxx_check_erase(__position);
        difference_type __offset = __position.base() - _Base::cbegin();
@@ -504,9 +515,11 @@ namespace __debug
       constexpr iterator
       erase(const_iterator __first, const_iterator __last)
       {
-       if (std::is_constant_evaluated())
+       if consteval
+         {
            return iterator(_Base::erase(__first.base(), __last.base()),
                            this);
+         }
 
        __glibcxx_check_erase_range(__first, __last);