[gcc r12-8267] libstdc++: Add std::atomic<shared_ptr>(nullptr_t) constructor (LWG 3661)

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:41ccd683f0002d7b5127aafc00dcd59e7837f0ab

commit r12-8267-g41ccd683f0002d7b5127aafc00dcd59e7837f0ab
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Apr 25 18:25:07 2022 +0100

    libstdc++: Add std::atomic<shared_ptr>(nullptr_t) constructor (LWG 3661)
    
    This DR was approved at the February 2022 plenary.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/shared_ptr_atomic.h (atomic<shared_ptr>): Add
            constructor for constant initialization from nullptr_t.
            * testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc:
            Check for new constructor.

Diff:
---
 libstdc++-v3/include/bits/shared_ptr_atomic.h                         | 4 ++++
 libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/libstdc++-v3/include/bits/shared_ptr_atomic.h b/libstdc++-v3/include/bits/shared_ptr_atomic.h
index 9e4df7da7f8..ff86432f0b4 100644
--- a/libstdc++-v3/include/bits/shared_ptr_atomic.h
+++ b/libstdc++-v3/include/bits/shared_ptr_atomic.h
@@ -573,6 +573,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       constexpr atomic() noexcept = default;
 
+      // _GLIBCXX_RESOLVE_LIB_DEFECTS
+      // 3661. constinit atomic<shared_ptr<T>> a(nullptr); should work
+      constexpr atomic(nullptr_t) noexcept : atomic() { }
+
       atomic(shared_ptr<_Tp> __r) noexcept
       : _M_impl(std::move(__r))
       { }
diff --git a/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc b/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
index 1f97224bf6a..a1902745a3e 100644
--- a/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
+++ b/libstdc++-v3/testsuite/20_util/shared_ptr/atomic/atomic_shared_ptr.cc
@@ -18,6 +18,8 @@
 
 // Check constexpr constructor.
 constinit std::atomic<std::shared_ptr<int>> a;
+// LWG 3661. constinit atomic<shared_ptr<T>> a(nullptr); should work
+constinit std::atomic<std::shared_ptr<int>> a2 = nullptr;
 
 void
 test_is_lock_free()