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Showing content from https://en.cppreference.com/w/cpp/language/../error/../language/../utility/declval.html below:

std::declval - cppreference.com

Helper template for writing expressions that appear in unevaluated contexts, typically the operand of decltype. In unevaluated context, this helper template converts any type T (which may be an incomplete type) to an expression of that type, making it possible to use member functions of T without the need to go through constructors.

std::declval can only be used in unevaluated contexts and is not required to be defined; it is an error to evaluate an expression that contains this function. Formally, the program is ill-formed if this function is odr-used.

(none)

Cannot be evaluated and thus never returns a value. The return type is T&& (reference collapsing rules apply) unless T is (possibly cv-qualified) void, in which case the return type is T.

std::declval is commonly used in templates where acceptable template parameters may have no constructor in common, but have the same member function whose return type is needed.

template<typename T>
typename std::add_rvalue_reference<T>::type declval() noexcept
{
    static_assert(false, "declval not allowed in an evaluated context");
}

#include <iostream>
#include <utility>
 
struct Default
{
    int foo() const { return 1; }
};
 
struct NonDefault
{
    NonDefault() = delete;
    int foo() const { return 1; }
};
 
int main()
{
    decltype(Default().foo())               n1 = 1;     // type of n1 is int
    decltype(std::declval<Default>().foo()) n2 = 1;     // same
 
//  decltype(NonDefault().foo())               n3 = n1; // error: no default constructor
    decltype(std::declval<NonDefault>().foo()) n3 = n1; // type of n3 is int
 
    std::cout << "n1 = " << n1 << '\n'
              << "n2 = " << n2 << '\n'
              << "n3 = " << n3 << '\n';
}

Output:

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