How does proposal N4502 work? (the "detection idiom")

Question

I was looking at the proposal N4502, and have been trying to wrap my head around it. I'm good up to section 5, but then what I thought I understood falls away. Perhaps it is just how I'm looking at it. Given the following:

// primary template handles all types not supporting the operation:
template< class, template<class> class, class = void_t< > >
struct
  detect : false_type { };
// specialization recognizes/validates only types supporting the archetype:
template< class T, template<class> class Op >
struct
  detect< T, Op, void_t<Op<T>> > : true_type { };

To use this detect metafunction, we supply it with another metafunction (i.e., a meta-callback) that fills the role of the archetypal expression. For example, here is an implementation of the is_assignable type trait:

// archetypal expression for assignment operation:
template< class T >
using
  assign_t = decltype( declval<T&>() = declval<T const &>() )
// trait corresponding to that archetype:
template< class T >
using
  is_assignable = detect<void, assign_t, T>;

I should be able to check if a type is assignable. No example of how it is used is given, so I'm assuming that it should be as easy as:

static_assert(is_assignable<int>::value, "Not assignable.");

Now just looking at this, doesn't look right. I don't see any way that assign_t is going to interact with the type T.

How this reads to me is:

is_assignable<int>
-> detect<void, assign_t, int>

Which will then fail to match any specialization and go to the base case which inherits from std::false_type.

Compiling this here seems to agree with my understanding.

So, what am I missing? How is this supposed to be used?

Answer 1

There is a typo, it should be

template< class T >
using is_assignable = detect<T, assign_t>;

Demo