How to tell if template type is an instance of a template class?

Question

I have a function that takes a template type to determine a return value. Is there any way to tell at compile time if the template type is some instantiation of a template c

Answer 1

Yet another improvement to the answer of @RichardHodges: Usually, one also wants to capture not only plain types, but rather all cv-qualified and ref-qualified types.

Put differently, if is_instance{} is true, also is_instance{} or is_instance{} should be true. The current implementations in this thread don't support that.

The following code accounts for all cv-ref-qualified types, by adding another indirection and a std::decay_t:

namespace
{
    template  typename>
    struct is_instance_impl : public std::false_type {};

    template