Matching variadic non-type templates

Question

Let\'s say I have two structs, Foo and Bar:

template
struct Foo{};

template
struct Bar{};
         


        

Answer 1

Question: What is a workaround for this in C++14?

A possible workaround in C++14 is based on traits.
As a minimal, working example (maybe even stupid, but it helps getting the idea):

#include 
#include 

template
struct Foo{};

template
struct Bar{};

template
struct traits;

template
struct traits> { using type = Foo<0>; };

template
struct traits> { using type = Bar<0>; };

template
constexpr bool match = std::is_same::type, typename traits::type>::value;

int main() {
    using f1 = Foo<1, 2, 3>;
    using f2 = Foo<1>;
    using b1 = Bar<1, 2, 3>;
    using b2 = Bar<1>;

    static_assert(match, "Fail");
    static_assert(match, "Fail");
    static_assert(!match, "Fail");
}

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As a side note, in C++17 you can simplify things up as it follows:

template class S, auto... U, auto... V>
struct match_class, S> : std::true_type{};

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About the reasons that are behind the error, @Barry's answer contains all what you need to understand it (as usual).