re PR libstdc++/64140 (match_results.prefix() returns an incorrect result if regex_iterator holds a zero-length match)
PR libstdc++/64140
* include/bits/regex.tcc (regex_iterator<>::operator++): Update
prefix.matched after modifying prefix.first.
* testsuite/28_regex/iterators/regex_iterator/char/64140.cc: New
testcase.
From-SVN: r218340
