https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99957
Bug ID: 99957
Summary: Ill-formed std::pair construction supported
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: minor
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
This shouldn't compile:
#include <utility>
struct move_only
{
move_only() = default;
move_only(move_only&&) = default;
};
move_only mo() { return {}; }
std::pair<void*, move_only> p0(0, mo());
std::pair<void*, move_only> p1({}, mo());
According to the standard, these constructors are considered:
pair(const first_type&, const second_type&);
template<class U1, class U2> pair(U1&&, U2&&);
For both p0 and p1, the first does not participate in overload resolution
because second_type is not copy constructible.
For p0, the second constructor does not participate because U1 is deduced as
int, and a pointer cannot be constructed from int. For p1, U1 cannot be deduced
from {}.
The code compiles with libstdc++ because we have additional non-standard
constructors, added by PR 40925 and described in
https://gcc.gnu.org/pipermail/libstdc++/2021-April/052299.html (where I
proposed to deprecate these non-standard constructors, and eventually remove
them).
