Currently, the return type of this lambda is decltype(auto), so the
lambda ends up returning a copy of _M_parent->_M_inner rather than a
reference to it when _S_ref_glvalue is false. This means _M_inner and
ranges::end(__inner_range) are respectively an iterator and sentinel for
different ranges, so comparing them is undefined.
libstdc++-v3/ChangeLog:
PR libstdc++/100290
* include/std/ranges (join_view::_Iterator::operator++): Correct
the return type of the lambda to avoid returning a copy of
_M_parent->_M_inner.
* testsuite/std/ranges/adaptors/join.cc (test10): New test.
85ef4b8d4e