[Bug c++/95050] New: coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[Bug c++/95050] New: coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[okannen at gmail dot com]

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]/§5.3
> [...] 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)

[Bug c++/95050] coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95050

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |iains at gcc dot gnu.org
   Target Milestone|---                         |10.2
   Last reconfirmed|                            |2020-05-14
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

[Bug c++/95050] coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95050

--- Comment #1 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Iain D Sandoe <iains@gcc.gnu.org>:

https://gcc.gnu.org/g:324276ff9b1aa5128e5cb9f5d43182d1ebab0752

commit r11-835-g324276ff9b1aa5128e5cb9f5d43182d1ebab0752
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Tue Jun 2 16:47:54 2020 +0100

    coroutines: Wrap co_await in a target expr where needed [PR95050]

    Since the co_await expression is mostly opaque to the existing
    machinery, we were hiding the details of the await_resume return
    value.  If that needs to be wrapped in a target expression, then
    emulate this with the whole co_await.  Similarly, if the await
    expression we build in response to co_await p.yield_value (e)
    is wrapped in a target expression, then we need to transfer that
    wrapper to the resultant CO_YIELD_EXPR (which is, itself, just
    a proxy for the underlying co_await).

    gcc/cp/ChangeLog:

            PR c++/95050
            * coroutines.cc (build_co_await): Wrap the co_await expression
            in a TARGET_EXPR, where needed.
            (finish_co_yield_expr): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/95050
            * g++.dg/coroutines/pr95050.C: New test.

[Bug c++/95050] coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95050

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Iain D Sandoe
<iains@gcc.gnu.org>:

https://gcc.gnu.org/g:ac9b05305d376cfa391dd57a46515cbdc165f094

commit r10-8261-gac9b05305d376cfa391dd57a46515cbdc165f094
Author: Iain Sandoe <iain@sandoe.co.uk>
Date:   Sun Jun 7 13:47:54 2020 +0100

    coroutines: Wrap co_await in a target expr where needed [PR95050]

    Since the co_await expression is mostly opaque to the existing
    machinery, we were hiding the details of the await_resume return
    value.  If that needs to be wrapped in a target expression, then
    emulate this with the whole co_await.  Similarly, if the await
    expression we build in response to co_await p.yield_value (e)
    is wrapped in a target expression, then we need to transfer that
    wrapper to the resultant CO_YIELD_EXPR (which is, itself, just
    a proxy for the underlying co_await).

    gcc/cp/ChangeLog:

            PR c++/95050
            * coroutines.cc (build_co_await): Wrap the co_await expression
            in a TARGET_EXPR, where needed.
            (finish_co_yield_expr): Likewise.

    gcc/testsuite/ChangeLog:

            PR c++/95050
            * g++.dg/coroutines/pr95050.C: New test.

    (cherry picked from commit 324276ff9b1aa5128e5cb9f5d43182d1ebab0752)

[Bug c++/95050] coroutine: no "mandatory copy elision" for prvalue await_resume expression.

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95050

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #3 from Iain Sandoe <iains at gcc dot gnu.org> ---
fixed on master and for 10.2