The libstdc++ manual documents that _T can not be used, because it's a
macro in system headers on some targets.
Backport from mainline
2020-05-01 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/94901
* include/std/type_traits (__is_complete_or_unbounded): Replace
BADNAME _T with _Tp.
* testsuite/17_intro/badnames.cc: New test.
