Why do my SFINAE expressions no longer work with GCC 8.2?

Question

I recently upgraded GCC to 8.2, and most of my SFINAE expressions have stopped working.

The following is somewhat simplified, but demonstrates the problem:

Answer 1

Partial answer: use typename = typename enable_if<...>, T=0 with different Ts:

#include 
#include 

class Class {
public:
    template <
        typename U,
        typename = typename std::enable_if_t<
            std::is_const::type>::value
        >, int = 0
    >
    void test() {
        std::cout << "Constant" << std::endl;
    }

    template <
        typename U,
        typename = typename  std::enable_if_t<
            !std::is_const::type>::value
        >, char = 0
    >
    void test() {
        std::cout << "Mutable" << std::endl;
    }
};

int main() {
    Class c;
    c.test();
    c.test();
    return 0;
}

(demo)

Still trying to figure out what the heck does std::enable_if<...>::type... mean knowing the default type is void.