[Bug c++/85743] New: Cannot call template member function inside a variadic lambda unless specifying `this`

[gufideg at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85743

            Bug ID: 85743
           Summary: Cannot call template member function inside a variadic
                    lambda unless specifying `this`
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gufideg at gmail dot com
  Target Milestone: ---

When using a particular pattern to expand a variadic template inside a member
function, `this->` need to be specified before calling other member functions.

Here is the most reduced repro I can come with at the moment:

    #include <type_traits>
    #include <tuple>

    template<typename>
    struct tuple_sequence;

    template<typename I, I... S>
    struct tuple_sequence<std::integer_sequence<I, S...>> {
        using type = std::tuple<std::integral_constant<I, S>...>;
    };

    template<typename... Types, typename F>
    constexpr auto apply_sequence_for(F function) -> decltype(auto) {
        return std::apply(function, typename
tuple_sequence<std::index_sequence_for<Types...>>::type{});
    }

    template<typename... Ts>
    struct Foo {
        template<typename T, typename...>
        constexpr void boom(int i) const {}

        constexpr auto foo(std::array<int, sizeof...(Ts)> data) const {
            apply_sequence_for<Ts...>([this, &data](auto... s) {
                (boom<Ts>(data[s]), ...);
            });
        }
    };

    int main() {
        Foo<int, float, char> f;
        f.foo({0, 1, 2});
    }


Notice the particular expansion pattern `boom<Ts>(data[s])`. The bug won't
trigger if the pattern is replaced by `boom<Ts...>(data[s])` or even
`boom<Ts>(data[0])`.


This code compiles under Clang 6.0.0
Live example: https://godbolt.org/g/n6wMUS