[Bug libstdc++/94160] std::pmr::pool_options::max_blocks_per_chunk=1 causes pool resources to return null pointers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94160

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Testcase:

#include <memory_resource>

int main()
{
  std::pmr::unsynchronized_pool_resource upr({ 1, 32 });
  auto* p = (int*)upr.allocate(4);
  *p = 0;
  return *p;
}

The returned pointer is null, leading to a segfault.

If libstdc++ is built with assertions enabled, the allocation aborts with:

../../../../../libstdc++-v3/src/c++17/memory_resource.cc:294:
std::pmr::{anonymous}::bitset::bitset(void*,
std::pmr::{anonymous}::bitset::size_type): Assertion 'empty()' failed.