void* MemoryMappedFile::map(....) {
  ....
  size_t len = strlen(filename);
  for (size_t i = len - 1; i >= 0; i--) {            // <=
    if (filename[i] == '/' || filename[i] == '\\')
      break;

    if (filename[i] == ':')
      filename[i] = '_';
  }
  ....
}

A link to the source code on GitHub

PVS-Studio warning: V547 Expression 'i >= 0' is always true. Unsigned type value is always >= 0. mmap_windows.cpp 197

The conditional statement of the loop (i >= 0) should always be true, as the loop counter ( i ) is of unsigned type (size_t).

This issue was originally reported in SERVER-28570.