[Bug c++/109323] New: No error when neither of return_value or return_void is defined

[enricomaria.dean6elis at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109323

            Bug ID: 109323
           Summary: No error when neither of return_value or return_void
                    is defined
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: enricomaria.dean6elis at gmail dot com
  Target Milestone: ---

Repro:

1. take the example here ->
https://en.cppreference.com/w/cpp/language/coroutines
2. remove the 3 `if (cond) co_return;` from `fibonacci_sequence`
3. remove `return_void` from `promise_type`
4. compile

Alternatively: https://godbolt.org/z/v3xd3b7TE, where both GCC and Clang
erroneously accept the code, whereas MSVC correcly rejects it.
(http://eel.is/c++draft/dcl.fct.def.coroutine#6)

I'm aware of https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95517, but that makes
no reference to the fact that not declaring any of the two functions is
erroneous.