PVS-Studio warning V547 for QT

bool equals( class1* val1, class2* val2 ) const{
{
  ...
  size_t size = val1->size();
  ...
  while ( --size >= 0 ){
    if ( !comp(*itr1,*itr2) )
      return false;
    itr1++;
    itr2++;
  }
  ...
}

An array overrun may surely occur.

This suspicious code was found in QT project by PVS-Studio static code analyzer.
Warning message is:
V547 Expression '-- size >= 0' is always true. Unsigned type value is always >= 0. QtCLucene arrays.h 154

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