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. 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.
