On 22/05/17 16:14 -0400, Tim Song wrote:
On Mon, May 22, 2017 at 9:21 AM, Jonathan Wakely <jwak...@redhat.com> wrote:
On 19/05/17 22:40 -0700, Tim Shen via libstdc++ wrote:
diff --git a/libstdc++-v3/include/std/variant
b/libstdc++-v3/include/std/variant
index 0e04a820d69..b9824a5182c 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -936,9 +936,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
noexcept((is_nothrow_move_constructible_v<_Types> && ...)) =
default;
template<typename _Tp,
+ typename = enable_if_t<!is_same_v<decay_t<_Tp>, variant>>,
typename =
enable_if_t<__exactly_once<__accepted_type<_Tp&&>>
- && is_constructible_v<__accepted_type<_Tp&&>,
_Tp&&>
- && !is_same_v<decay_t<_Tp>, variant>>>
+ && is_constructible_v<__accepted_type<_Tp&&>,
_Tp&&>>>
Does this definitely short-circuit? I seem to recall a similar case
where either Clang or GCC (I think it was Clang) was evaluating the
second default template argument even though the first had produce a
substition failure.
If we need to guarantee it short-circuits then we'd want:
template<typename _Tp,
typename = enable_if_t<__and_<
__not_<is_same<decay_t<_Tp>, variant>>,
__bool_constant<
__exactly_once<__accepted_type<_Tp&&>>
&& is_constructible_v<__accepted_type<_Tp&&>,
_Tp&&>>>
i.e. use __and_< is-this-type, everything-else> where
"everything-else" still uses && to avoid making the instantiations too
deep.
Also, this is another place where we could use an __is_samey<T, U>
trait that does is_same<remove_cv_t<remove_reference_t<T>, U>.
The thing that needs to be short-circuited out is the evaluation of
__accepted_type<_Tp&&>, which starts the tower of turtles.
Ah I see.
The original patch does that (assuming core issue 1227's resolution),
but the __and_ version doesn't; __and_ only short circuits the
immediate parameter, not things used in forming it.
Then the original patch is OK for trunk and gcc-7-branch.
Thank you Tim and Tim for the explanations.