The Standard says

> If searches for the names return_void and return_value in the scope
of the promise type each find any declarations, the program is ill-
formed.

However, there are cases where one wants both "co_return;" and
co_return something;" statements in the same coroutine.

The cases are when the coroutine returns a sum type such as std::future
or std::expected, and one of the options is void (e.g.
std::future<void> or std::expected<void, something>. In these cases,
one would want


  co_return;

to select the void branch of the sum type

and

  co_return std::make_exception_ptr(...) // for std::future<void>

or

  co_return something(...)  // for std::expected<void, something>.


to select the non-void branch.

Early versions of gcc did not error when both return_value() and
return_void() were defined; no problems were observed.


Proposal:
 - remove this requirement from the Standard.

Alternatives:
 1. Co-opt co_await to return (suggested by this list some time ago);
this is what I use now in Seastar. It's unsatisfying to use co_await
when you mean to return.
 2. Use co_return some_dummy_type() to force both branches to use
return_value(). Again unsatisfying to force users to type out a dummy
type when they want a non-erroneous return; also forces co_return when
falling off the end.
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