std::type_identity(3)

NAME
std::type_identity - std::type_identity

Synopsis
   Defined in header <type_traits>
   template< class T > (since C++20)
   struct type_identity;
   Provides the member typedef type that names T (i.e., the identity transformation).
   If the program adds specializations for std::type_identity, the behavior is
   undefined.

Member types
   Name Definition
   type T

Helper types
   template< class T > (since C++20)
   using type_identity_t = type_identity<T>::type;

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

Notes
   std::type_identity can be used to establish non-deduced contexts in template
   argument deduction.
     Feature-test macro Value Std Feature
   __cpp_lib_type_identity 201806L (C++20) std::type_identity

Example
// Run this code
 #include <iostream>
 #include <type_traits>
 template<class T>
 T foo(T a, T b) { return a + b; }
 template<class T>
 T bar(T a, std::type_identity_t<T> b) { return a + b; }
 int main()
 {
     // foo(4.2, 1); // error, deduced conflicting types for 'T'
     std::cout << bar(4.2, 1) << '\n'; // OK, calls bar<double>
 }

Output:
 5.2

See also
   identity function object that returns its argument unchanged
   (C++20) (class)