pmr::resource_adaptor can avoid allocating an oversized buffer and doing
manual alignment within that buffer when the wrapped allocator is known
to always meet the requested alignment. Specifically, if the allocator
is known to use malloc or new directly, then we can call the allocator
directly for any fundamental alignment.
PR libstdc++/70940
* include/experimental/memory_resource
(__resource_adaptor_common::_AlignMgr::_M_unadjust): Add assertion.
(__resource_adaptor_common::__guaranteed_alignment): New helper to
give maximum alignment an allocator guarantees. Specialize for known
allocators using new and malloc.
(__resource_adaptor_imp::do_allocate): Use __guaranteed_alignment.
(__resource_adaptor_imp::do_deallocate): Likewise.
* testsuite/experimental/memory_resource/new_delete_resource.cc:
Check that new and delete are called with expected sizes.
From-SVN: r262935
