[Bug c++/103358] New: what is the first constructor argument of lambda coroutine promise_type?

[Bug c++/103358] New: what is the first constructor argument of lambda coroutine promise_type?

[netcan1996 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103358

            Bug ID: 103358
           Summary: what is the first constructor argument of lambda
                    coroutine promise_type?
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: netcan1996 at gmail dot com
  Target Milestone: ---

the C++20 standard(N4858) says:
> promise-constructor-arguments is determined as follows: overload resolution is performed on a promise
constructor call created by assembling an argument list with lvalues p1 . . .
pn. If a viable constructor is
found (12.4.3), then promise-constructor-arguments is (p1 , . . . , pn),
otherwise promise-constructor-
arguments is empty.

I tested gcc-12 coroutine feature, when I use the free coroutine function,
arguments are that function, but when I use lambda coroutine, the first
argument looks strange, the rest arguments are the same as lambda.

```cpp
auto test = [](int, char, double) -> task<void> {
    co_return;
};

```

the promise_type dumps all arguments type:
```cpp
        template<typename ...T>
        promise_type(T&&... args) {
            dump<T...> {};
        }
```

T = {const<lambda(int, char, double)>&&, int&, char&, double&} here, what is
the type `const<lambda(int, char, double)>&&`?

[Bug c++/103358] what is the first constructor argument of lambda coroutine promise_type?

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103358

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |iains at gcc dot gnu.org

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
There was a long discussion about this between the various WG21 paper authors.

The stated intend was that 'this' pointer and the 'lambda object pointer' were
intended to be cast to references (to the same type as their pointers).

This behaviour should be the same between MSVC, clang and GCC (but I fear that
the amendment did not make C++20 timescale - so we might need a coordinated
change to remove it from the compilers).

Does that explain what you are seeing?

[Bug c++/103358] what is the first constructor argument of lambda coroutine promise_type?

[netcan1996 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103358

--- Comment #2 from Net Can <netcan1996 at gmail dot com> ---
(In reply to Iain Sandoe from comment #1)
> There was a long discussion about this between the various WG21 paper
> authors.
> 
> The stated intend was that 'this' pointer and the 'lambda object pointer'
> were intended to be cast to references (to the same type as their pointers).
> 
> This behaviour should be the same between MSVC, clang and GCC (but I fear
> that the amendment did not make C++20 timescale - so we might need a
> coordinated change to remove it from the compilers).
> 
> Does that explain what you are seeing?

yes, I see. thank you for your explanation.