[Bug c++/81700] New: Unresolved function type when taking address of operator() of generic lambda

["gufideg at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81700

            Bug ID: 81700
           Summary: Unresolved function type when taking address of
                    operator() of generic lambda
           Product: gcc
           Version: 7.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gufideg at gmail dot com
  Target Milestone: ---

Hi, have found this corner case when taking the address of operator() of a
generic lambda. This error only happen when using decltype with a function
defined as follow:

    // Take a type, return that same type
    template<typename T>
    T identity(T);

    int main()
    {
        auto f = [](auto test) {};

        using F = decltype(f);

        // Error! Unresolved function type?
        (void)decltype(identity(&F::operator()<int>)){};
    }

However, taking the address of the lambda beforehand make GCC accept the code:

    // Take a type, return that same type
    template<typename T>
    T identity(T);

    int main()
    {
        auto f = [](auto test) {};

        using F = decltype(f);

        // We simply add this line
        (void)decltype(&F::operator()<int>){};

        // Compiles!
        (void)decltype(identity(&F::operator()<int>)){};
    }

Here's a live example: http://coliru.stacked-crooked.com/a/b7909605e2f882c9

Note that this bug don't happen with non-generic lambdas.