https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95050
Bug ID: 95050
Summary: coroutine: no "mandatory copy elision" for prvalue
await_resume expression.
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: okannen at gmail dot com
Target Milestone: ---
Checked with gcc version 10.1.1 20200508 (GCC) (see bellow for the `gcc -v`
output)
The result of the await-resume expression is not directly used to initialize
the result of the co_await expression.
This has the observable side effect to cause a (move) copy construction when
the result the await-resume expression is a prvalue.
I think the pertinent standard paragraph is, [expr.await]/ยง5.3
> [...] the await-resume expression is evaluated, and its result is the result
> of the await-expression.
The code bellow compiles on clang but fail to compile on gcc:
#ifdef __clang__
#include <experimental/coroutine>
using namespace std::experimental;
#else
#include <coroutine>
using namespace std;
#endif
struct task
{
struct promise_type
{
auto get_return_object () -> task
{
return {};
}
auto initial_suspend () -> suspend_never
{
return {};
}
auto final_suspend () -> suspend_always
{
return {};
}
void unhandled_exception ()
{ }
void return_void ()
{}
};
};
struct ret_type
{
ret_type () = default;
ret_type (const ret_type&) =delete;
};
struct awaiter
{
auto await_ready() const -> bool
{
return true;
}
void await_suspend (coroutine_handle<>)
{}
auto await_resume() -> ret_type
{
return {};
}
};
task f()
{
ret_type r2 {co_await awaiter{}};
};
int main()
{
auto x = f();
return 0;
}
This code was tentatively compiled with `c++ -std=c++20 -fcoroutines`
`gcc -v`:
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/olivier/usr/libexec/gcc/x86_64-pc-linux-gnu/10/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --enable-libsanitizer
--prefix=/home/olivier/usr/ --with-gcc-major-version-only --disable-bootstrap
--enable-language=c,c++,lto
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.1.1 20200508 (GCC)
