[Bug c++/95517] New: [coroutines] suggested warning when co_return and return_void() are missing

[bruck.michael at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95517

            Bug ID: 95517
           Summary: [coroutines] suggested warning when co_return and
                    return_void() are missing
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bruck.michael at gmail dot com
  Target Milestone: ---

[stmt.return.coroutine]
"3 If p.return_void() is a valid expression, flowing off the end of a coroutine
is equivalent to a co_return with no operand; otherwise flowing off the end of
a coroutine results in undefined behavior."

It would be useful to diagnose this.

Example code that includes the undefined behavior and produces no warnings with
-Wall -Wextra:
---
#include <coroutine>

struct dummy
{
    struct promise_type
    {
        dummy get_return_object() const noexcept { return {}; }
        std::suspend_never initial_suspend() const noexcept { return {}; }
        std::suspend_never final_suspend() const noexcept { return {}; }
        //void return_void() const noexcept {}
        void unhandled_exception() const noexcept {}
    };
    int i; // work around #95516
};

dummy foo()
{ 
    co_await std::suspend_never{};
    //co_return;
}

int main()
{
    foo();
}