[dcl.fct.def.coroutine] q1 in a lambda whose `operator()` is a coroutine

[frederick-vs-ja]

Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [dcl.fct.def.coroutine]

Link to reflector thread (if any):

Issue description:

CWG2465 clarified that the q₁ is an lvalue that denotes *this for an implicit object member function. However, in a lambda expression whose operator() is an implicit object member function and a coroutine, *this doesn't denote the lambda closure object, and is possibly invalid (when neither this nor *this is captured).

Perhaps for a lambda expression we should specify q₁ to be an lvalue that denotes the lambda closure object, and is const unless the lambda is mutable.

However, it seem that a captureless lambda should be handled as if the operator() is a static even if it's an implicit object member function. Otherwise, there would be behavioral difference between the operator() and the converted function pointer, as the latter can never have *this.

Suggested resolution: