The current constexpr implementation of std::char_traits<C>::move relies
on being able to compare the pointer parameters, which is not allowed
for unrelated pointers. We can use __builtin_constant_p to determine
whether it's safe to compare the pointers directly. If not, then we know
the ranges must be disjoint and so we can use char_traits<C>::copy to
copy forwards from the first character to the last. If the pointers can
be compared directly, then we can simplify the condition for copying
backwards to just two pointer comparisons.
libstdc++-v3/ChangeLog:
PR libstdc++/113200
* include/bits/char_traits.h (__gnu_cxx::char_traits::move): Use
__builtin_constant_p to check for unrelated pointers that cannot
be compared during constant evaluation.
* testsuite/21_strings/char_traits/requirements/113200.cc: New
test.
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