Public bug reported:
Release: Ubuntu 26.04.1 LTS (resolute)
Package: g++-15 15.2.0-16ubuntu1
g++-15 emits a false-positive -Wmaybe-uninitialized warning when a structured
binding is initialized from a co_await expression in a coroutine. With
-Werror, this breaks the build of valid C++20 code, in particular the common
Boost.Asio idiom:
auto [ec, result] = co_await
op(boost::asio::as_tuple(boost::asio::use_awaitable));
This is an upstream GCC 15 regression that is already fixed on the GCC 16
branch:
- GCC 14.3: no warning
- GCC 15.2: warning (also this package, 15.2.0-16ubuntu1)
- GCC 16.1: warning
- GCC 16.2: no warning (also Debian forky, g++ 16.2.0-3)
(-std=c++20 -O1 -Wall on all of them.)
The fix landed upstream on releases/gcc-16 between 16.1 and 16.2. Since 26.04
is an LTS release that keeps GCC 15 as its default compiler, could the fix be
backported to gcc-15 once it is identified upstream?
Upstream report: https://gcc.gnu.org/pipermail/gcc-help/2026-October/145199.html
(gcc-help thread; no GCC Bugzilla PR yet)
Steps to reproduce:
1. Save the following as repro.cpp:
#include <coroutine>
#include <memory>
#include <tuple>
#include <utility>
struct task {
struct promise_type {
std::shared_ptr<int> value;
task get_return_object() { return {}; }
std::suspend_always initial_suspend() noexcept { return {}; }
std::suspend_always final_suspend() noexcept { return {}; }
void return_value(std::shared_ptr<int> v) { value = std::move(v); }
void unhandled_exception() noexcept {}
};
};
extern std::coroutine_handle<> pending;
struct op {
std::tuple<int, std::shared_ptr<int>> result;
bool await_ready() const noexcept { return false; }
void await_suspend(std::coroutine_handle<> h) noexcept { pending = h; }
std::tuple<int, std::shared_ptr<int>> await_resume() { return
std::move(result); }
};
op async_op();
task f()
{
auto [ec, r] = co_await async_op();
co_return r;
}
2. Run:
g++-15 -std=c++20 -O1 -Wall -c repro.cpp
Expected result: no warning (the binding is always initialized).
Actual result:
repro.cpp: In function 'void f(_Z1fv.Frame*)':
repro.cpp:32:1: warning: '<anonymous>' may be used uninitialized
[-Wmaybe-uninitialized]
32 | }
| ^
repro.cpp:30:38: note: '<anonymous>' was declared here
30 | auto [ec, r] = co_await async_op();
| ^
Further observations on 15.2.0-16ubuntu1:
- warns at -O1, -O2, -O3 and -Os; no warning at -O0;
- same result with -std=c++20, -std=c++23 and -std=c++26;
- no warning when the tuple is a named variable instead of a structured
binding:
auto t = co_await async_op(); auto& r = std::get<1>(t);
- no warning for the same structured binding in a regular (non-coroutine)
function.
Workaround: -Wno-maybe-uninitialized.
** Affects: gcc-15 (Ubuntu)
Importance: Undecided
Status: New
--
You received this bug notification because you are a member of Ubuntu
Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2170751
Title:
-Wmaybe-uninitialized false positive on structured binding initialized
from co_await (fixed in GCC 16.2)
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/gcc-15/+bug/2170751/+subscriptions
--
ubuntu-bugs mailing list
[email protected]
https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
