Compiling this:

template < class T, class Y, class ...Args >
struct isSame
{
    static constexpr bool value = std::conditional<
                                      sizeof...( Args ),
                                      typename std::conditional<
                                          std::is_same< T, Y >::value,
                                          isSame< Y, Args... >, // Error!
                                          std::false_type >::type,
                                      std::is_same< T, Y > >::type::value;
};

int main()
{
    qDebug() << isSame< double, int >::value;
    return EXIT_SUCCESS;
}

Gives me this compiler error:

error: wrong number of template arguments (1, should be 2 or more)

The issue is that isSame< double, int > has an empty Args parameter pack, so isSame< Y, Args... > effectively becomes isSame< Y > which does not match the signature.

But my question is: Why is that branch being evaluated at all? sizeof...( Args ) is false, so the inner std:conditional should not be evaluated. This isn't a runtime piece of code, the compiler knows that sizeof..( Args ) will never be true with the given template types.

If you're curious, it's supposed to be a variadic version of std::is_same, not that it works...