Std::declval
Converts any type T to a reference type, making it possible to use member functions in decltype expressions without the need to go through constructors.
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.
Note that because no definition exists for declval, it can only be used in unevaluated contexts; it is an error to evaluate an expression that contains this function. Formally, the program is ill-formed if this function is odr-used.