libstdc++: Replace reserved identifier _T with _Tp (PR 94901)

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.

libstdc++-v3/ChangeLog

+2020-05-01  Jonathan Wakely  <jwakely@redhat.com>
+
+	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.
+
 2020-04-30  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/89510

libstdc++-v3/include/std/type_traits

@@ -197,8 +197,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   // Helper functions that return false_type for incomplete classes,
   // incomplete unions and arrays of known bound from those.
 
-  template <typename _T, size_t = sizeof(_T)>
-    constexpr true_type __is_complete_or_unbounded(__type_identity<_T>)
+  template <typename _Tp, size_t = sizeof(_Tp)>
+    constexpr true_type __is_complete_or_unbounded(__type_identity<_Tp>)
     { return {}; }
 
   template <typename _TypeIdentity,

libstdc++-v3/testsuite/17_intro/badnames.cc

+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-do compile { target x86_64-*-linux* } }
+
+// Names taken from coding_style.bad_identifiers in the libstdc++ manual.
+// We can't test this on all targets, because these names are used in
+// non-GCC system headers. Test on x86_64-linux where we know these particular
+// names aren't used.
+
+// For Solaris:
+#define _B		_B is a BADNAME
+#define _C		_C is a BADNAME
+#define _L		_L is a BADNAME
+#define _N		_N is a BADNAME
+#define _P		_P is a BADNAME
+#define _S		_S is a BADNAME
+#define _U		_U is a BADNAME
+#define _X		_X is a BADNAME
+#define _E1		_E1 is a BADNAME
+#define _E2		_E2 is a BADNAME
+#define _E3		_E3 is a BADNAME
+#define _E4		_E4 is a BADNAME
+#define _E5		_E5 is a BADNAME
+#define _E6		_E6 is a BADNAME
+#define _E7		_E7 is a BADNAME
+#define _E8		_E8 is a BADNAME
+#define _E9		_E9 is a BADNAME
+#define _E10		_E10 is a BADNAME
+#define _E11		_E11 is a BADNAME
+#define _E12		_E12 is a BADNAME
+#define _E13		_E13 is a BADNAME
+#define _E14		_E14 is a BADNAME
+#define _E15		_E15 is a BADNAME
+#define _E16		_E16 is a BADNAME
+#define _E17		_E17 is a BADNAME
+#define _E18		_E18 is a BADNAME
+#define _E19		_E19 is a BADNAME
+#define _E20		_E20 is a BADNAME
+#define _E21		_E21 is a BADNAME
+#define _E22		_E22 is a BADNAME
+#define _E23		_E23 is a BADNAME
+#define _E24		_E24 is a BADNAME
+
+// Irix adds:
+#define _A		_A is a BADNAME
+#define _G		_G is a BADNAME
+
+// MS adds:
+#define _T		_T is a BADNAME
+
+// BSD adds:
+#define __used		__used is a BADNAME
+#define __unused	__unused is a BADNAME
+        // __inline	(glibc uses this so can't test here)
+        // _Complex	(glibc uses this so can't test here)
+#define __istype	__istype is a BADNAME
+#define __maskrune	__maskrune is a BADNAME
+#define __tolower	__tolower is a BADNAME
+#define __toupper	__toupper is a BADNAME
+#define __wchar_t	__wchar_t is a BADNAME
+#define __wint_t	__wint_t is a BADNAME
+        // _res
+        // _res_ext
+	// __tg_*
+
+// VxWorks adds:
+#define _C2		_C2 is a BADNAME
+
+
+#include <bits/stdc++.h>