[ADT] Simplify a constructor of iterator_range (NFC) (#158005)

Without this patch, we determine whether one iterator type can be
converted to another in a roundabout way.  Specifically,
explicitly_convertible uses std::void_t to determine whether the given
conversion expression is well formed, yielding
std::true_type/std::false_type.  Then the boolean value is passed to
std::enable_if_t to obtain void again.  That is, we are doing a
roundtrip from void to a boolean value and back.

This patch removes the roundtrip by directly using std::void_t inside
the constructor's template parameter list.

Now, explicitly_converted_t is very similar to std::is_constructible,
but there a couple of corner cases where they evaluate to different
values.  For now, this patch sticks to the same expression
decltype(static_cast<To>(...)) to be safe.

0 files changed, 0 insertions, 0 deletions