libstdc++: Make ranges::advance(it, n, bound) follow standard more strictly

The standard specifies some of the effects of ranges::advance in terms
of "Equivalent to:" and it's observable that our current implementation
deviates from the precise specification in the standard.  This was
causing some failures in the libc++ testsuite.

For the sized_sentinel_for<I, S> case I optimized our implementation to
avoid redundant calls when we have already checked that there's nothing
to do.  We were eliding `advance(i, bound)` when the iterator already
equals the sentinel, and eliding `advance(i, n)` when `n` is zero. In
both cases, removing the seemingly redundant calls is not equivalent to
the spec because `i = std::move(bound)` or `i += 0` operations can be
observed by program-defined iterators. This patch inlines the observable
side effects of advance(i, bound) or advance(i, 0) without actually
calling those functions.

For the non-sized sentinel case, `if (i == bound || n == 0)` is
different from `if (n == 0 || i == bound)` for the case where n is zero
and a program-defined iterator observes the number of comparisons.
This patch changes it to do `n == 0` first. I don't think this is
required by the standard, as this condition is not "Equivalent to:" any
observable sequence of operations, but testing `n == 0` first is
probably cheaper anyway.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (ranges::advance(i, n, bound)):
	Ensure that observable side effects on iterators match what is
	specified in the standard.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

0 files changed, 0 insertions, 0 deletions