Non-deduced context like 'boost::mpl::identity<T>::type' in standard library?

Question

Cosider the following example I digged up here on StackOverflow

  template<typename T, typename Pred> 
  T const & clamp ( T const& val, 
    typename boost::mpl::identity<T>::type const & lo, 
    typename boost::mpl::identity<T>::type const & hi, Pred p )
  {
//    assert ( !p ( hi, lo ));    // Can't assert p ( lo, hi ) b/c they might be equal
    return p ( val, lo ) ? lo : p ( hi, val ) ? hi : val;
  } 

where typename boost::mpl::identity<T>::type prevents the compiler from deducing T based on the type of the second and the third argument. This comes very handy for me, but I cannot use the Boost Library (please do not give me a hard time on that, as it is already a hard time because of that).

The question is now is something equivallent in the standard library directly which I just cannot find?

Answer 1

C++20 will have std::type_identity. But you don't really have to wait for the standard library to have it. Its entire implementation is:

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

template< class T >
using type_identity_t = typename type_identity<T>::type;