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).
   The behavior of a program that adds specializations for type_identity is undefined.

Member types
   Name Definition
   type T

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

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

Notes
   type_identity can be used to establish non-deduced contexts in template argument
   deduction.
   Feature-test macro: __cpp_lib_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