[Bug c++/82632] New: copy deduction candidate erroneously preferred over deduction-guide

[barry.revzin at gmail dot com]

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].