https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96784
Bug ID: 96784
Summary: Templated lambda body remains constexpr despite having
co_await in body
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: kacper.slominski72 at gmail dot com
Target Milestone: ---
When a templated lambda has a fold expression with co_await inside of it in
it's body, it remains constexpr which causes a compile error. When compiling
the following code (with "-std=c++20 -fcoroutines -Wall -Wextra -c"):
#include <coroutine>
#include <utility>
#include <cstddef>
struct dummy_coro {
using promise_type = dummy_coro;
bool await_ready() { return false; }
void await_suspend(std::coroutine_handle<>) { }
void await_resume() { }
dummy_coro get_return_object() { return {}; }
dummy_coro initial_suspend() { return {}; }
dummy_coro final_suspend() { return {}; }
void return_void() { }
void unhandled_exception() { }
};
dummy_coro foo() {
co_await []<size_t ...I>(std::index_sequence<I...>) -> dummy_coro {
((co_await [](int){ return std::suspend_never{}; }(I)), ...);
}(std::make_index_sequence<1>{});
}
The following error is produced:
test.cpp: In instantiation of
‘_Z3foov.actor(foo()::_Z3foov.frame*)::<lambda(std::index_sequence<I ...>)>
[with long unsigned int ...I = {0}; std::index_sequence<I ...> =
std::integer_sequence<long unsigned int, 0>]’:
test.cpp:20:33: required from here
test.cpp:19:5: error: ‘co_await’ cannot be used in a ‘constexpr’ function
19 | ((co_await [](int){ return std::suspend_never{}; }(I)), ...);
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The error goes away when the body also contains a co_return/co_await/co_yield
that is not in a fold expression.
