Invocable requirement for CPOs is incorrect

[CaseyCarter]

[customization.point.object]/4 states:

The type of a customization point object T shall satisfy Invocable<const T, Args...> (7.6.2) 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.

This requires const rvalue T to have such a function call operator, instead of const lvalue T as intended. The wording should require the call operator for lvalue T, ideally without forbidding the presence of equivalent call operators for rvalue (or non-const, or volatile, or ...) T`.

Proposed Resolution

Modify [customization.point.object]/4 as follows:

 The type of a customization point object T shall satisfy
-Invocable<const T, Args...>
+Invocable<const T&, Args...>
 (22.6.2) when the types of Args... meet the requirements specified in that customization point
 object’s definition.
-Otherwise,
+When the types of Args... do not meet the customization point object’s requirements,
 T shall not have a function call operator that participates in overload resolution.

[CaseyCarter]

Note that this PR has already been applied to the CPO wording in D0898R0.

[timsong-cpp]

There seems to be another problem here: the type of an object includes its cv-qualification, if any, but for something like the Semiregular requirement (and this Invocable) presumably we want to remove the cv-qualification.

[ericniebler]

I've moved the issue with the Semiregular-ness of CPOs to #621. Closing this one.