libstdc++: Value-initialize std::atomic for C++20 (P0883R2)

This implements the new requirements for C++20 that std::atomic should
initialize the atomic variable in its default constructor.

This patch does not add the deprecated attribute to atomic_init, but
that should be done at some point as it's deprecated in C++20.

The paper also deprecates the ATOMIC_FLAG_INIT macro, although we can't
apply the deprecated attribute to a macro.

	PR libstdc++/58605
	* include/bits/atomic_base.h (__cpp_lib_atomic_value_initialization):
	Define.
	(__atomic_flag_base, __atomic_base, __atomic_base<_PTp*>)
	(__atomic_float): Add default member initializer for C++20.
	* include/std/atomic (atomic): Likewise.
	(atomic::atomic()): Remove noexcept-specifier on default constructor.
	* include/std/version (__cpp_lib_atomic_value_initialization): Define.
	* testsuite/29_atomics/atomic/cons/assign_neg.cc: Adjust dg-error line
	number.
	* testsuite/29_atomics/atomic/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/cons/value_init.cc: New test.
	* testsuite/29_atomics/atomic_flag/requirements/trivial.cc: Adjust
	expected result for is_trivially_default_constructible.
	* testsuite/29_atomics/atomic_float/requirements.cc: Likewise.
	* testsuite/29_atomics/atomic_float/value_init.cc: New test.
	* testsuite/29_atomics/atomic_integral/cons/assign_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/copy_neg.cc: Likewise.
	* testsuite/29_atomics/atomic_integral/cons/value_init.cc
	* testsuite/29_atomics/atomic_integral/requirements/trivial.cc: Adjust
	expected results for is_trivially_default_constructible.
	* testsuite/util/testsuite_common_types.h (has_trivial_dtor): Add
	new test generator.

11 files changed, 202 insertions, 7 deletions

diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/cons/assign_neg.cc b/libstdc++-v3/testsuite/29_atomics/atomic/cons/assign_neg.cc
index 306ea5a..d93eacc 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/cons/assign_neg.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/assign_neg.cc
@@ -27,5 +27,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 639 }
+// { dg-error "deleted" "" { target *-*-* } 659 }
 // { dg-prune-output "include" }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/cons/copy_neg.cc b/libstdc++-v3/testsuite/29_atomics/atomic/cons/copy_neg.cc
index 5032912..03ecdd4 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic/cons/copy_neg.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/copy_neg.cc
@@ -27,5 +27,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 678 }
+// { dg-error "deleted" "" { target *-*-* } 698 }
 // { dg-prune-output "include" }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic/cons/value_init.cc b/libstdc++-v3/testsuite/29_atomics/atomic/cons/value_init.cc
new file mode 100644
index 0000000..8620c7c
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/cons/value_init.cc
@@ -0,0 +1,76 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+
+#ifndef __cpp_lib_atomic_value_initialization
+# error "Feature test macro for atomic value-initialization is missing"
+#elif __cpp_lib_atomic_value_initialization < 201911L
+# error "Feature test macro for atomic value-initialization has wrong value"
+#endif
+
+#include <testsuite_hooks.h>
+
+struct A
+{
+  constexpr A() : val(42) { }
+  int val;
+};
+
+constexpr std::atomic<A> a;
+
+void
+test01()
+{
+  VERIFY(a.load().val == 42);
+  static_assert(!std::is_nothrow_default_constructible_v<std::atomic<A>>);
+}
+
+struct B
+{
+  constexpr B() noexcept : val(99) { }
+  int val;
+};
+
+constexpr std::atomic<B> b;
+
+void
+test02()
+{
+  VERIFY(b.load().val == 99);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<B>>);
+}
+
+constexpr std::atomic<int*> c;
+
+void
+test03()
+{
+  VERIFY(c.load() == nullptr);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<int*>>);
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_flag/cons/value_init.cc b/libstdc++-v3/testsuite/29_atomics/atomic_flag/cons/value_init.cc
new file mode 100644
index 0000000..547e406
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_flag/cons/value_init.cc
@@ -0,0 +1,37 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+void
+test01()
+{
+  std::atomic_flag f;
+  VERIFY(!f.test_and_set());
+  VERIFY(f.test_and_set());
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic_flag>);
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_flag/requirements/trivial.cc b/libstdc++-v3/testsuite/29_atomics/atomic_flag/requirements/trivial.cc
index d08adf8..ecf7ace 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_flag/requirements/trivial.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_flag/requirements/trivial.cc
@@ -22,6 +22,10 @@
 
 void test01()
 {
+#if __cplusplus <= 201703L
   __gnu_test::has_trivial_cons_dtor test;
+#else
+  __gnu_test::has_trivial_dtor test;
+#endif
   test.operator()<std::atomic_flag>();
 }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/requirements.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/requirements.cc
index 1599655..b8a5562 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_float/requirements.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/requirements.cc
@@ -25,7 +25,7 @@ test01()
 {
   using A = std::atomic<float>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, float> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );
@@ -41,7 +41,7 @@ test02()
 {
   using A = std::atomic<double>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, double> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );
@@ -57,7 +57,7 @@ test03()
 {
   using A = std::atomic<long double>;
   static_assert( std::is_standard_layout_v<A> );
-  static_assert( std::is_trivially_default_constructible_v<A> );
+  static_assert( !std::is_trivially_default_constructible_v<A> );
   static_assert( std::is_trivially_destructible_v<A> );
   static_assert( std::is_same_v<A::value_type, long double> );
   static_assert( std::is_same_v<A::difference_type, A::value_type> );
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/value_init.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/value_init.cc
new file mode 100644
index 0000000..237c0dd
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/value_init.cc
@@ -0,0 +1,37 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+constexpr std::atomic<double> a;
+
+void
+test01()
+{
+  VERIFY(a.load() == 0);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<double>>);
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/assign_neg.cc b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/assign_neg.cc
index d842761..3c164b4 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/assign_neg.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/assign_neg.cc
@@ -28,5 +28,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 639 }
+// { dg-error "deleted" "" { target *-*-* } 659 }
 // { dg-prune-output "include" }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/copy_neg.cc b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/copy_neg.cc
index a83a214..0131ba2 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/copy_neg.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/copy_neg.cc
@@ -28,5 +28,5 @@ int main()
   return 0;
 }
 
-// { dg-error "deleted" "" { target *-*-* } 678 }
+// { dg-error "deleted" "" { target *-*-* } 698 }
 // { dg-prune-output "include" }
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/value_init.cc b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/value_init.cc
new file mode 100644
index 0000000..fa1eb7a
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_integral/cons/value_init.cc
@@ -0,0 +1,37 @@
+// 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-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+constexpr std::atomic<int> a;
+
+void
+test01()
+{
+  VERIFY(a.load() == 0);
+  static_assert(std::is_nothrow_default_constructible_v<std::atomic<int>>);
+}
+
+int
+main()
+{
+  test01();
+}
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_integral/requirements/trivial.cc b/libstdc++-v3/testsuite/29_atomics/atomic_integral/requirements/trivial.cc
index aac1218..36aa248 100644
--- a/libstdc++-v3/testsuite/29_atomics/atomic_integral/requirements/trivial.cc
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_integral/requirements/trivial.cc
@@ -22,7 +22,11 @@
 
 void test01()
 {
+#if __cplusplus <= 201703L
   __gnu_test::has_trivial_cons_dtor test;
+#else
+  __gnu_test::has_trivial_dtor test;
+#endif
   __gnu_cxx::typelist::apply_generator(test,
 				       __gnu_test::atomic_integrals::type());
 }