[Bug c++/106110] New: Expected ambiguity but it resolves fine

[Bug c++/106110] New: Expected ambiguity but it resolves fine

[ted at lyncon dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106110

            Bug ID: 106110
           Summary: Expected ambiguity but it resolves fine
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ted at lyncon dot se
  Target Milestone: ---

I made a type trait to test if a call to a function using certain arguments
would be ambiguous or not, but in the below example, where I expected that
calling the function `foo` with a `std::tuple<single_type>` would result in
ambiguity, it is not detected as such.
```
#include <iostream>
#include <tuple>

template <class T>
void foo(const std::tuple<T>&) { std::cout << "T\n"; }

template <class T, class... Ts>
void foo(const std::tuple<T, Ts...>&) { std::cout << "Ts...\n"; }

template <class... Args>
struct is_foo_call_ambiguous {
    static std::true_type test(...);

    template <class... AArgs>
    static auto test(AArgs&&...)
        -> decltype(foo(std::declval<AArgs>()...), std::false_type{});

    static constexpr bool value =
        decltype(test(std::declval<Args>()...))::value;
};

template <class F, class... Args>
static constexpr bool is_foo_call_ambiguous_v =
    is_foo_call_ambiguous<F, Args...>::value;

int main() {
    // expected `true` here, but it reports `false`:
    std::cout << is_foo_call_ambiguous_v<std::tuple<int>> << '\n';

    // `false` as expected:
    std::cout << is_foo_call_ambiguous_v<std::tuple<int, int>> << '\n';
}
```
As can be seen here, clang++ reports it to be ambiguous:
https://godbolt.org/z/YTevWvbzz

If I understand it correctly, the added tiebreaker
(https://cplusplus.github.io/CWG/issues/1395.html) for variadic templates
should not apply here.

[Bug c++/106110] Expected ambiguity but it resolves fine

[ted at lyncon dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106110

--- Comment #1 from Ted Lyngmo <ted at lyncon dot se> ---
Sorry, the helper variable template should be:
```
template <class... Args>
static constexpr bool is_foo_call_ambiguous_v =
    is_foo_call_ambiguous<Args...>::value;
```
It gives the same result: https://godbolt.org/z/bKbn8Gre7