std:: as_const

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Defined in header <utility>
template < class T >
constexpr std:: add_const_t < T > & as_const ( T & t ) noexcept ;
(1) (since C++17)
template < class T >
void as_const ( const T && ) = delete ;
(2) (since C++17)
1) Forms lvalue reference to const type of t .
2) const rvalue reference overload is deleted to disallow rvalue arguments.

Possible implementation 

template<class T>
constexpr std::add_const_t<T>& as_const(T& t) noexcept
{
    return t;
}

Notes

Feature-test macro Value Std Feature
__cpp_lib_as_const 201510L (C++17) std::as_const

Example

Run this code 
#include <cassert>
#include <string>
#include <type_traits>
#include <utility>
 
int main()
{
    std::string mutableString = "Hello World!";
    auto&& constRef = std::as_const(mutableString);
 
    mutableString.clear(); // OK
//  constRef.clear(); // Error: 'constRef' is 'const' qualified,
                      //        but 'clear' is not marked const
 
    assert(&constRef == &mutableString);
    assert(&std::as_const(mutableString) == &mutableString);
 
    using ExprType = std::remove_reference_t<decltype(std::as_const(mutableString))>;
 
    static_assert(std::is_same_v<std::remove_const_t<ExprType>, std::string>,
                  "ExprType should be some kind of string.");
    static_assert(!std::is_same_v<ExprType, std::string>,
                  "ExprType shouldn't be a mutable string.");
}

See also

(C++11)
checks if a type is const-qualified
(class template)
(C++11) (C++11) (C++11)
adds const and/or volatile specifiers to the given type
(class template)
(C++11) (C++11) (C++11)
removes const and/or volatile specifiers from the given type
(class template)
(C++23)
converts a view into a constant_range
(class template) (range adaptor object)
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