AFAICT, Cpp17InputIterators are not required to be default constructible,
since that requirement is added in Cpp17ForwardIterator. Hence, our
archetype for Cpp17InputIterator should not be default constructible.
Removing that constructor has a ripple effect on a couple of tests that
were making incorrect assumptions. Notably:
- Some tests were using cpp17_input_iterator as a sentinel for itself.
That is not valid, because a cpp17_input_iterator is not semiregular
anymore after the change (and hence it doesn't satisfy sentinel_for).
- Some tests were using a stride-counted cpp17_input_iterator as the
sentinel for a range. This doesn't work anymore because of the problem
above, so these tests were changed not to check stride counts for
input iterators.
- Some tests were default constructing cpp17_input_iterator when a simple
alternative was available -- those have been changed to use that alternative.
Differential Revision: https://reviews.llvm.org/D115806
4a47ac7d51