PVS-Studio warning V610 for tesseract-ocr

void Dawg::init(....)
{
  ....
  letter_mask_ = ~(~0 << flag_start_bit_);
  ....
}

This suspicious code was found in tesseract-ocr project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '~0' is negative. libtesseract303 dawg.cpp 187

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