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.
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