A recently-approved C++2a feature, trivial to implement.
* include/std/type_traits (type_identity, type_identity_t): Define
for C++2a.
* testsuite/20_util/type_identity/requirements/alias_decl.cc: New.
* testsuite/20_util/type_identity/requirements/
explicit_instantiation.cc:New.
* testsuite/20_util/type_identity/requirements/typedefs.cc: New.
Tested powerpc64le-linux, committed to trunk.
commit 54f6b78dc937cbaee50522bf9cd18ac4e0855d39
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Mon Jul 2 21:32:24 2018 +0100
P0887R1 The identity metafunction
* include/std/type_traits (type_identity, type_identity_t): Define
for C++2a.
* testsuite/20_util/type_identity/requirements/alias_decl.cc: New.
* testsuite/20_util/type_identity/requirements/
explicit_instantiation.cc:New.
* testsuite/20_util/type_identity/requirements/typedefs.cc: New.
diff --git a/libstdc++-v3/include/std/type_traits
b/libstdc++-v3/include/std/type_traits
index 0c7e97286ca..b2d3380f024 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2996,6 +2996,13 @@ template <typename _From, typename _To>
template<typename _Tp>
using remove_cvref_t = __remove_cvref_t<_Tp>;
+ /// Identity metafunction.
+ template<typename _Tp>
+ struct type_identity { using type = _Tp; };
+
+ template<typename _Tp>
+ using type_identity_t = typename type_identity<_Tp>::type;
+
#endif // C++2a
_GLIBCXX_END_NAMESPACE_VERSION
diff --git
a/libstdc++-v3/testsuite/20_util/type_identity/requirements/alias_decl.cc
b/libstdc++-v3/testsuite/20_util/type_identity/requirements/alias_decl.cc
new file mode 100644
index 00000000000..6729d118273
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/type_identity/requirements/alias_decl.cc
@@ -0,0 +1,35 @@
+// Copyright (C) 2018 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+//
+#include <type_traits>
+
+using namespace std;
+
+template<typename T, typename = std::type_identity_t<T>>
+ struct test; // undefined
+
+template<typename T>
+ struct test<T, T> : std::true_type { };
+
+static_assert( test<const int>{}, "type_identity_t<const int>" );
+static_assert( test<volatile int>{}, "type_identity_t<volatile int>" );
+static_assert( test<unsigned>{}, "type_identity_t<unsigned>" );
+static_assert( test<char>{}, "type_identity_t<char>" );
+static_assert( test<signed char>{}, "type_identity_t<signed char>" );
diff --git
a/libstdc++-v3/testsuite/20_util/type_identity/requirements/explicit_instantiation.cc
b/libstdc++-v3/testsuite/20_util/type_identity/requirements/explicit_instantiation.cc
new file mode 100644
index 00000000000..07387e719ce
--- /dev/null
+++
b/libstdc++-v3/testsuite/20_util/type_identity/requirements/explicit_instantiation.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2018 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// NB: This file is for testing type_traits with NO OTHER INCLUDES.
+
+#include <type_traits>
+
+namespace std
+{
+ typedef short test_type;
+ template struct type_identity<test_type>;
+}
diff --git
a/libstdc++-v3/testsuite/20_util/type_identity/requirements/typedefs.cc
b/libstdc++-v3/testsuite/20_util/type_identity/requirements/typedefs.cc
new file mode 100644
index 00000000000..77f0cba9b4c
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/type_identity/requirements/typedefs.cc
@@ -0,0 +1,94 @@
+// Copyright (C) 2018 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-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <type_traits>
+
+template<typename T, typename = typename std::type_identity<T>::type>
+ struct test; // undefined
+
+template<typename T>
+ struct test<T, T> : std::true_type { };
+
+enum test_enum { };
+struct test_class { };
+struct incomplete_class;
+
+void test01()
+{
+ static_assert(test<float>::value, "");
+ static_assert(test<const unsigned int>::value, "");
+ static_assert(test<const unsigned int>::value, "");
+ static_assert(test<volatile unsigned int>::value, "");
+ static_assert(test<const volatile unsigned int>::value, "");
+ static_assert(test<const unsigned char>::value, "");
+
+#ifdef _GLIBCXX_USE_WCHAR_T
+ static_assert(test<volatile wchar_t>::value, "" );
+#endif
+
+ // Pointers
+ static_assert(test<void*>::value, "");
+ static_assert(test<long*>::value, "");
+ // References
+ static_assert(test<short&>::value, "");
+ static_assert(test<char&&>::value, "");
+ static_assert(test<int*&>::value, "");
+ // Arrays
+ static_assert(test<int[]>::value, "");
+ static_assert(test<int[2]>::value, "");
+ static_assert(test<int[2][3]>::value, "");
+ static_assert(test<int(*)[2]>::value, "");
+ static_assert(test<int(&)[2]>::value, "");
+
+ static_assert(test<test_enum>::value, "");
+ static_assert(test<test_class>::value, "");
+ static_assert(test<incomplete_class>::value, "");
+
+ // Functions
+ static_assert(test<void(*)()>::value, "");
+ static_assert(test<int(*)(int) noexcept>::value, "");
+ static_assert(test<void(&)()>::value, "");
+ static_assert(test<long(&)(long) noexcept>::value, "");
+ static_assert(test<void()>::value, "");
+ static_assert(test<int(int, int)>::value, "");
+ static_assert(test<void() noexcept>::value, "");
+ static_assert(test<int(int, int) noexcept>::value, "");
+
+ // Abominable function types
+ static_assert(test<void(int) const>::value, "");
+ static_assert(test<void(int) const volatile>::value, "");
+
+ // Pointers to members
+ static_assert(test<int incomplete_class::*>::value, "");
+ static_assert(test<void(incomplete_class::*)(int)>::value, "");
+ static_assert(test<void(incomplete_class::*)(int) noexcept>::value, "");
+ static_assert(test<void(incomplete_class::*)() const>::value, "");
+ static_assert(test<void(incomplete_class::*)() &>::value, "");
+ static_assert(test<void(incomplete_class::*)() &&>::value, "");
+ static_assert(test<void(incomplete_class::*)() volatile &&>::value, "");
+
+#ifndef __STRICT_ANSI__
+ // GNU Extensions.
+#ifdef _GLIBCXX_USE_INT128
+ static_assert(test<unsigned __int128>::value, "");
+ static_assert(test<unsigned __int128>::value, "");
+#endif
+#endif
+}
