This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

2465. Coroutine parameters passed to a promise constructor

[Accepted as a DR at the June, 2021 meeting.]

The resolution of issue 2436 (in P2107R0) deleted the sentence

A reference to a parameter in the function-body of the coroutine and in the call to the coroutine promise constructor is replaced by a reference to its copy.

replacing it with new wording in 7.5.5.2 [expr.prim.id.unqual] paragraph 1:

An identifier that names a coroutine parameter refers to the copy of the parameter (9.5.4 [dcl.fct.def.coroutine]).

This new approach no longer covers coroutine parameters passed to a promise constructor, since the constructor call is implicit, as described in 7.5.5.2 [expr.prim.id.unqual] paragraph 5.

Suggested resolution:

1. Change 7.5.5.2 [expr.prim.id.unqual] paragraph 4 as follows:

In the following, p_i is an lvalue of type P_i, where p₁ denotes *this and p_i+1 denotes the i^th function parameter for a non-static member function, and p_i denotes the i^th function parameter otherwise. Let q_i be the corresponding parameter copy, as described below.

2. Change 7.5.5.2 [expr.prim.id.unqual] bullet 5.7 as follows:

A coroutine behaves as if its function-body were replaced by...

• ...

• promise-constructor-arguments is determined as follows: overload resolution is performed on a promise constructor call created by assembling an argument list with lvalues pq₁ ... pq_n. If a viable constructor is found (12.2.3 [over.match.viable]), then promise-constructor-arguments is (pq₁, ... , pq_n), otherwise promise-constructor-arguments is empty.

Proposed resolution (April, 2021):

1. Change 9.5.4 [dcl.fct.def.coroutine] paragraph 4 as follows:

In the following, p_i is an lvalue of type P_i, where p₁ denotes *this and p_i+1 denotes the i^th function parameter for a non-static member function, and p_i denotes the i^th function parameter otherwise. For a non-static member function, q₁ is an lvalue that denotes *this; any other q_i is an lvalue that denotes the parameter copy corresponding to p_i, as described below.

2. Change 9.5.4 [dcl.fct.def.coroutine] bullet 5.7 as follows:

A coroutine behaves as if its function-body were replaced by: ... where

• ...

• 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 ... p_n q₁ ... q_n. If a viable constructor is found (12.2.3 [over.match.viable]), then promise-constructor-arguments is (p1, ... , p_n) (q₁, ... , q_n), otherwise promise-constructor-arguments is empty.