https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82632
Bug ID: 82632
Summary: copy deduction candidate erroneously preferred over
deduction-guide
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Minimal example courtesy of STL from
https://bugs.llvm.org/show_bug.cgi?id=34970:
template <typename X, typename Y> struct IsSame {
static constexpr bool value = false;
};
template <typename Z> struct IsSame<Z, Z> {
static constexpr bool value = true;
};
template <typename T> struct Optional {
template <typename U> Optional(U&&) { }
};
template <typename A> Optional(A) -> Optional<A>;
int main() {
Optional opt(1729);
Optional dupe(opt);
static_assert(IsSame<decltype(opt), Optional<int>>::value);
static_assert(IsSame<decltype(dupe), Optional<int>>::value);
static_assert(!IsSame<decltype(dupe), Optional<Optional<int>>>::value);
}
gcc accepts this code, but dupe should be an Optional<Optional<int>> since the
deduction-guide should take precedence over the copy deduction candidate per
the ordering in [over.match.best].
