PVS-Studio warning V629 for tesseract-ocr

uinT64 letter_mask_;

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:
V629 Consider inspecting the '~0 << flag_start_bit_' expression. Bit shifting of the 32-bit value with a subsequent expansion to the 64-bit type. libtesseract303 dawg.cpp 187

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