https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69717
Bug ID: 69717
Summary: std::pair is incompatible with std::is_constructible
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: david at doublewise dot net
Target Milestone: ---
#include <type_traits>
#include <utility>
struct S {
S(int) {}
};
int main() {
using pair_t = std::pair<S, S>;
static_assert(std::is_default_constructible<pair_t>::value, "");
pair_t p;
}
The static_assert does not fire, even though it should. If I comment out line
11, everything compiles. With line 11 in, I get a compile error that
essentially tells me that pair_t is not default constructible. This is with
-std=c++1z, which has pair's constructors only participating in overload
resolution if they are valid.
Compiling with clang and libstdc++ gives different behavior. The static_assert
fails to compile, but not with a regular static_assert message. It gives a hard
error, similar to what gcc gives when you actually try to compile line 11.
However, it is a hard error not in the immediate context, so attempting to use
it for SFINAE with enable_if causes a compile time error. In other words, I get
a compiler error with clang if I just mention
std::is_default_constructible<pair_t>, which seems like 'correct' behavior
based on the implementation of std::pair (but not correct in c++1z mode).
So this actually seems like a bug in both gcc and libstdc++. libstdc++ has a
std::pair implementation that causes a hard error even in c++1z mode if you
check if it is default constructible and it is not, and gcc has a bug that
causes it to ignore that error and just return true for
std::is_default_constructible.
This looks related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68430
