Question

struct X{
    template<class T>
    X(){}
};

Is it possible to instantate such a type?

Answer 1

Yes, it is possible to have such a constructor, but it’s impossible to call it. All the template parameters of a templated constructor must be deduced from the parameter list or have a default value. In Your example you can’t instantiate the class.

[temp.mem]

[ Note: Because the explicit template argument list follows the function template name,
and because conversion member function templates and constructor member function templates are called
without using a function name, there is no way to provide an explicit template argument list for these
function templates. —end note ]