Describe the bug

Commit c40a251 (fixing issue #4091, merged into master in issue #4072) introduced an endless loop into deque::shrink_to_fit(). In specific constellations in regards to _First_used_block_idx, _First_unused_block_idx and _Mask the first loop for deallocating unused blocks runs infinetely.

Command-line test case

#define _CRT_SECURE_NO_WARNINGS

#include <windows.h>
#include <math.h>
#include <deque>

int main()
{
  std::deque<int> qu;

  long it = 0;
  while (1)
  {
    size_t numAlloc = rand() + 1;
    for (size_t i = 0; i < numAlloc; i++)
    {
      qu.push_back(0);
    }

    size_t numDealloc = rand() + 1;
    if (it % 100 == 0 || numDealloc > qu.size())
    {
      numDealloc = qu.size();
    }
    for (size_t i = 0; i < numDealloc; i++)
    {
      qu.pop_front();
    }
    qu.shrink_to_fit();

    printf("iteration %d: %lld\n", ++it, qu.size());
  }

  return 0;
}

After about 40 iterations deque::shrink_to_fit get's stuck.

Expected behavior

Termination condition for loop is correct so deque::shrink_to_fit() terminates.

STL version

• Visual Studio version

  Microsoft Visual Studio Enterprise 2022 
  Version 17.10
  

• Visual Studio version

  Microsoft Visual Studio Enterprise 2022 
  Version 17.11.1