clang vs gcc - empty generic lambda variadic argum

问题:

I think I found another "clang vs gcc" inconsistency between lambdas and callable objects.

decltype(l)::operator() should be equivalent to C::operator(), but if variadic pack is left empty in the generic lambda, gcc refuses to compile:

15 : error: no match for call to '(main()::) (int)' l(1);

15 : note: candidate: decltype (((main()::)0u).main()::(x, )) (*)(auto:1&&, auto:2&&, ...)

15 : note: candidate expects 3 arguments, 2 provided

14 : note: candidate: template main()::

auto l = [](auto&& x, auto&&...) { return x; };

14 : note: template argument deduction/substitution failed:

15 : note: candidate expects 2 arguments, 1 provided

l(1);

Live example on godbolt.org.

struct C
{
    template<typename T, typename... TRest>
    auto operator()(T&& x, TRest&&...){ return x; }
};

int main()
{
    // Compiles both with clang and gcc.
    auto c = C{};
    c(1);

    // Compiles with clang 3.7.
    // Does not compile with gcc 5.2.
    auto l = [](auto&& x, auto&&...) { return x; };
    l(1);
}

Could not find anything related to this on the gcc bug tracker (didn't spend too much time searching though) - is gcc wrong here?

回答1:

I've reported the issue as gcc bug #68071.