[Bug c++/101992] New: new object was created erroneously when co_await on function call that return lvalue reference

[lbqq0000 at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101992

            Bug ID: 101992
           Summary: new object was created erroneously when co_await on
                    function call that return lvalue reference
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lbqq0000 at gmail dot com
  Target Milestone: ---

Created attachment 51326
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51326&action=edit
preprocessed file showing the issue

gcc-11 can't compile the bellow code:

$ cat test.cxx
#include <coroutine>

using namespace std;

struct awaitable {
    awaitable() = default;
    awaitable(const awaitable&) = delete;
    awaitable(const awaitable&&) = delete;

    bool await_ready() { return false; }
    void await_suspend(coroutine_handle<> h) { }
    void await_resume() {}
};

awaitable& request();

struct task{
    struct promise_type {
        task get_return_object() { return {}; }
        suspend_never initial_suspend() { return {}; }
        suspend_never final_suspend() noexcept { return {}; }
        void return_void() {}
        void unhandled_exception() {}
    };
};

task test_coroutine() {
    co_await request();
}

$ g++-11 -v
Using built-in specs.
COLLECT_GCC=g++-11
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.1.0-1ubuntu1~21.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-RPS7jb/gcc-11-11.1.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-RPS7jb/gcc-11-11.1.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (Ubuntu 11.1.0-1ubuntu1~21.04) 


The error message is:
$ g++-11 -std=c++20 -c test.cxx
test.cxx: In function ‘task test_coroutine()’:
test.cxx:28:22: error: use of deleted function ‘awaitable::awaitable(const
awaitable&&)’
   28 |     co_await request();
      |                      ^
test.cxx:8:5: note: declared here
    8 |     awaitable(const awaitable&&) = delete;
      |     ^~~~~~~~~

The reason is that the co_await expression is trying to create a new object and
use the result of request() to move-construct it.
This behavior is not compliant with the standard. In chapter 7.6.2.3 note 3.4
is saying that:
e is an lvalue referring to the result of evaluating the (possibly-converted)
o.
So when the above function call returned an lvalue reference the e should be a
reference to the o not a new object.