Ambiguous template instantiation

Question

Can someone explain the ambiguity here?

template <typename...> struct thing;

template <typename... Rest>
struct thing<int&, Rest&...> {
    thing(int&, Rest&...) { }
};

template <typename First, typename... Rest>
struct thing<First&, Rest&...> {
    thing(First&, Rest&...) { }
};

int main() {
    int myint;
    char mychar;
    thing<int&, char&> t(myint, mychar);
}

well, which thing are you referring to? The one that takes 1 + other args, or the one that takes 2 + other args? — UKMonkey, Nov 27 '17 at 13:56
Isn't the first specialization more specialized than the second? It should take the first one automatically, right? — prestokeys, Nov 27 '17 at 13:57
GCC 7.2 compains. It works when I remove the references from the types though. — prestokeys, Nov 27 '17 at 14:01
Reminds me of https://stackoverflow.com/questions/40892048/class-template-specialization-priority-ambiguity — Rerito, Nov 27 '17 at 14:05
Any workaround here if I must get this to work with GCC 7.2? Perhaps using `std::enable_if_t<!std::is_same_v>* = nullptr` somewhere? — prestokeys, Nov 27 '17 at 14:18
You can have `template struct thing : thing_for_ref {}` (and then specialize `thing_for_ref`) as workaround. — Jarod42, Nov 27 '17 at 14:26
A working workaround using SFINAE: https://wandbox.org/permlink/cPGy0K4I5BR2EWIm — Rerito, Nov 27 '17 at 14:30
The latest msvc (19.11.25547) also has no problems, at least after the variables are initialized. — SoronelHaetir, Nov 27 '17 at 16:10

score 0 · Answer 1 · answered Oct 23 '18 at 08:13

if you specialize on int instead of int& it works

template <typename...> struct thing;

template <typename... Rest>
struct thing<int, Rest...> {
    thing(int&, Rest&...) { }
};

template <typename First, typename... Rest>
struct thing<First, Rest...> {
    thing(First&, Rest&...) { }
};

Ambiguous template instantiation

1 Answers1