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From: "oremanj at mit dot edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112360] New: [coroutines] unreachable 'co_await' still
 creates a suspension point
Date: Fri, 03 Nov 2023 06:30:24 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112360

            Bug ID: 112360
           Summary: [coroutines] unreachable 'co_await' still creates a
                    suspension point
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: oremanj at mit dot edu
  Target Milestone: ---

A coroutine function of the form:

    task example() {
        do {
            co_await awaitable{};
        } while (false && co_await awaitable{});
        co_return;
    }

will execute the awaitable's await_ready() and await_suspend() methods twic=
e,
even though execution of the abstract machine only reaches one co_await
statement. This reproduces on several tested versions from 11.1 to current
master, at either of optimization level -O0 or -O2. While the construct may
seem a bit nonsensical, it does show up in practice; it's simplified from
something that appears after macro expansion when using the popular Catch2
testing library if you write an assertion about the value of a co_await
expression.

Compiler Explorer link: https://godbolt.org/z/EoP7EnEnf

Full test case is below. On gcc it prints:
    ready
    suspend
    resume
    ready
    suspend
    returned
    final suspend

clang (14 through 17) shows the more expected
    ready
    suspend
    resume
    returned
    final suspend

command line: gcc -std=3Dc++20 -Wall -Wextra t.c -o t

--- test case follows ---

#include <coroutine>

extern "C" int puts(const char* s);

struct task {
    struct promise_type {
        std::suspend_always initial_suspend() { return {}; }
        std::suspend_always final_suspend() noexcept {
            puts("final suspend");
            return {};
        }
        void return_void() { puts("returned"); }
        void unhandled_exception() {}
        task get_return_object() { return task{this}; }
    };

    promise_type* promise;
};

struct awaitable {
    bool await_ready() const noexcept { puts("ready"); return false; }
    void await_suspend(std::coroutine_handle<> h) {
        puts("suspend");
        h.resume();
    }
    bool await_resume() {
        puts("resume");
        return false;
    }
};

task example() {
    do {
        co_await awaitable{};
    } while (false && co_await awaitable{});
    co_return;
}

int main() {
    using handle =3D std::coroutine_handle<task::promise_type>;
    task t =3D example();
    handle h =3D handle::from_promise(*t.promise);
    h.resume();
}=