The constraints upon, and latitude of, implementations of the Ranges library follow the same constraints and latitudes for implementations of the C++ standard library as specified in ISO/IEC 14882:2014 §[conforming].
A customization point object is a function object ([function.objects]) with a literal class type that interacts with user-defined types while enforcing semantic requirements on that interaction.
The type of a customization point object shall satisfy Semiregular ([concepts.lib.object.semiregular]).
All instances of a specific customization point object type shall be equal ([concepts.lib.general.equality]).
The type of a customization point object T shall satisfy Invocable<const T, Args...> ([concepts.lib.callable.invocable]) when the types of Args... meet the requirements specified in that customization point object's definition. Otherwise, T shall not have a function call operator that participates in overload resolution.
Each customization point object type constrains its return type to satisfy a particular concept.
The library defines several named customization point objects. In every translation unit where such a name is defined, it shall refer to the same instance of the customization point object.
[ Note: Many of the customization point objects in the library evaluate function call expressions with an unqualified name which results in a call to a user-defined function found by argument dependent name lookup ( ISO/IEC 14882:2014 §[basic.lookup.argdep]). To preclude such an expression resulting in a call to unconstrained functions with the same name in namespace std, customization point objects specify that lookup for these expressions is performed in a context that includes deleted overloads matching the signatures of overloads defined in namespace std. When the deleted overloads are viable, user-defined overloads must be more specialized ( ISO/IEC 14882:2014 §[temp.func.order]) or more constrained (Concepts TS [temp.constr.order]) to be used by a customization point object. — end note ]