[#metafunction]
[section Template metafunction]

A ['template metafunction] represents a function over types that is evaluated at
compile-time. It is implemented by a template class.

The template arguments of that class are expected to be types (`class` or
`typename` template arguments). They represent the arguments of the
metafunction.

Instances of the template class are expected to have a public nested type called
`type`. This type is the type the metafunction returns.

Template metafunction are expected to be called with
[link metaprogramming_value template metaprogramming values] as arguments only.

Template metafunctions are expected to return template metaprogramming values.

For example this is the identity template metafunction:

  template <class T>
  struct identity
  {
    using type = T;
  };

This metafunction is called `identity`. It takes one argument, `T`. The result
of calling this metafunction is its argument, `T`.

To call this metafunction, one has to instantiate the template class. The
template arguments it is instantiated with are the arguments the metafunction is
called with. For example:

  identity<std::integral_constant<int, 13>>::type

The above example calls the metafunction `identity` with
`std::integral_constant<int, 13>` as its argument.

[endsect]