PVS-Studio warning V610 for Clang

int find_next(unsigned Prev) const {
  ...
  // Mask off previous bits.
  Copy &= ~0L << BitPos;
  ...
}

This suspicious code was found in Clang project by PVS-Studio static code analyzer.
Warning message is:
V610 Undefined behavior. Check the shift operator '<<. The left operand '~0L' is negative. bitvector.h 175

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