Both OnceAction compatibility traits, IsDirectlyCompatible and
IsCompatibleAfterIgnoringArguments, check std::is_constructible before
is_callable_r.
On libc++ that order breaks. Evaluating
is_constructible<std::tuple<OnceAction&&>, ...> drives libc++'s tuple
constructor SFINAE, which calls back into OnceAction's converting
constructor and re-enters the conjunction while it is still incomplete.
GCC <= 8 (e.g. QNX 7.1's gcc 8.3) rejects this with:
incomplete type ... used in nested name specifier
so EXPECT_CALL(m, f()).WillOnce(Return(...)) fails to compile. GCC >= 9
and every Clang accept the original order; libstdc++ is unaffected.
Swapping the two checks fixes it: is_callable_r is cheap and
non-recursive, so the conjunction short-circuits to false for the
non-callable tuple before is_constructible is instantiated.
Fixes#3947.
