libstdc++: Add C++20 std::make_shared enhancements (P0674R1)

This adds the overloads of std::make_shared and std::allocate_shared for
creating arrays, added to C++20 by P0674R1.

It also adds std::make_shared_for_overwrite, added to C++20 by P1020R1
(and renamed by P1973R1). The std::make_unique_for_overwite overloads
are already supported.

The original std::make_shared overload is changed to construct a
shared_ptr directly instead of calling std::allocate_shared. This
removes a function call at runtime, and avoids having to do overload
resolution for std::allocate_shared, now that there are five overloads
of it.

Allocating a shared array is done by a new __shared_count constructor.
An array is allocated with space for additional elements at the end and
an instance of new _Sp_counted_array class template is constructed in
that unused capacity.

The non-array form of std::make_shared_for_overwrite uses the same
__shared_count constructor as the original std::make_shared overload,
but a new partial specialization of _Sp_counted_ptr_inplace is selected
when the allocator's value_type is the new _Sp_overwrite_tag type. That
new partial specialization default-initializes its contained object and
destroys it with a destructor call rather than using the allocator.

Despite being C++20 features, this implementation only uses concepts
conditionally, with workarounds when they are not supported. This allows
it to work with older non-GCC compilers (Clang 9 and icc 2021). At some
point we can simplify the code by removing the workarounds.

libstdc++-v3/ChangeLog:

	* include/bits/shared_ptr.h (__cpp_lib_shared_ptr_weak_type):
	Correct type of macro value.
	(shared_ptr): Add additional friend declarations.
	(make_shared, allocate_shared): Constrain existing overloads and
	remove static_assert.
	* include/bits/shared_ptr_base.h (__cpp_lib_smart_ptr_for_overwrite):
	New macro.
	(_Sp_counted_ptr_inplace<T, Alloc, Lp>): New partial
	specialization for use with make_shared_for_overwrite.
	(__cpp_lib_shared_ptr_arrays): Update value for C++20.
	(_Sp_counted_array_base): New class template.
	(_Sp_counted_array): New class template.
	(__shared_count(_Tp*&, const _Sp_counted_array_base&, _Init)):
	New constructor for allocating shared arrays.
	(__shared_ptr(const _Sp_counted_array_base&, _Init)): Likewise.
	* include/std/version (__cpp_lib_shared_ptr_weak_type): Correct
	type.
	(__cpp_lib_shared_ptr_arrays): Update value for C++20.
	(__cpp_lib_smart_ptr_for_overwrite): New macro.
	* testsuite/20_util/shared_ptr/creation/99006.cc: Adjust
	expected errors.
	* testsuite/20_util/shared_ptr/creation/array.cc: New test.
	* testsuite/20_util/shared_ptr/creation/overwrite.cc: New test.
	* testsuite/20_util/shared_ptr/creation/version.cc: New test.
	* testsuite/20_util/unique_ptr/creation/for_overwrite.cc: Check
	feature test macro. Test non-trivial default-initialization.

1 files changed, 5 insertions, 1 deletions

diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index c5d5efb..a8b792e 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -51,8 +51,10 @@
 
 #if _GLIBCXX_HOSTED
 # define __cpp_lib_allocator_traits_is_always_equal 201411
+#if __cplusplus < 201703L // N.B. updated value in C++20
 # define __cpp_lib_shared_ptr_arrays 201611L
 #endif
+#endif
 
 #if !defined(__STRICT_ANSI__)
 // gnu++11
@@ -167,7 +169,7 @@
 # define __cpp_lib_scoped_lock 201703
 # define __cpp_lib_shared_mutex 201505L
 #endif
-#define __cpp_lib_shared_ptr_weak_type 201606
+#define __cpp_lib_shared_ptr_weak_type 201606L
 #define __cpp_lib_string_view 201803L
 #if _GLIBCXX_HAVE_USELOCALE
 # define __cpp_lib_to_chars 201611L
@@ -275,7 +277,9 @@
 #if __cpp_lib_atomic_wait || _GLIBCXX_HAVE_POSIX_SEMAPHORE
 # define __cpp_lib_semaphore 201907L
 #endif
+#define __cpp_lib_shared_ptr_arrays 201707L
 #define __cpp_lib_shift 201806L
+#define __cpp_lib_smart_ptr_for_overwrite 202002L
 #if __cpp_lib_concepts
 # define __cpp_lib_span 202002L
 #endif