By making __glibcxx_assert constexpr compatible we can get rid of a 
FIXME in basic_string_view and so fix following XPASS in _GLIBCXX_DEBUG 
modes.

XPASS: 21_strings/basic_string_view/element_access/char/2.cc execution test
XPASS: 21_strings/basic_string_view/element_access/wchar_t/2.cc 
execution test

Should I also rename those in 2_neg.cc ?

I had to move the assert block in c++config to benefit from 
_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED definition. The generation 
of the compilation error is not very nice but it is still better than no 
error and we can see the assertion:

/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:31:22: 
erreur: condition non constante pour l'assertion statique
    31 | static_assert(test() == 0); // { dg-error "non-constant 
condition" }
       |               ~~~~~~~^~~~
Dans le fichier inclus depuis 
/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/limits:42,
                  depuis 
/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/string_view:40,
                  depuis 
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:21:
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:31:19: 
dans l'expansion « constexpr » de « test() »
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:28:13: 
dans l'expansion « constexpr » de 
« s.std::basic_string_view<wchar_t>::operator[](4) »
/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/string_view:206:2: 
erreur: inline assembly is not a constant expression
   206 |  __glibcxx_assert(__pos < this->_M_len);
       |  ^~~~~~~~~~~~~~~~
/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/string_view:206:2: 
note: only unevaluated inline assembly is allowed in a « constexpr » 
function in C++2a

             * include/bits/c++config
[_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED](__glibcxx_assert_impl):
             New.
             * include/std/string_view 
(basic_string_view<>:operator[](size_type)):
             Restore __glibcxx_assert.
             (basic_string_view<>::front()): Likewise.
             (basic_string_view<>::back()): Likewise.
             * 
testsuite/21_strings/basic_string_view/element_access/char/back_constexpr_neg.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/char/constexpr.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/char/constexpr_neg.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/char/front_back_constexpr.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/char/front_constexpr_neg.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr.cc:
             New.
             * 
testsuite/21_strings/basic_string_view/element_access/wchar_t/constexpr_neg.cc:
             New.

Tested under Linux x86_64 normal and debug modes.

Ok to commit ?

François