This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 110b. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2023-01-15


2585. Name lookup for coroutine allocation

Section: 9.5.4  [dcl.fct.def.coroutine]     Status: CD6     Submitter: Xu Chuanqi     Date: 2022-05-12

[Accepted at the July, 2022 meeting.]

Consider:

  struct Allocator;

  struct resumable::promise_type {
    void* operator new(std::size_t sz, Allocator&);
    // ...
  };
  resumable foo() {
    co_return;
  }

Subclause 9.5.4 [dcl.fct.def.coroutine] paragraph 9 specifies:

... The allocation function's name is looked up by searching for it in the scope of the promise type. If no viable function is found (12.2.3 [over.match.viable]), overload resolution is performed again on a function call created by passing just the amount of space required as an argument of type std::size_t.

Is the example ill-formed because resumable::promise_type is not viable, or is the example well-formed because the global operator new can be used? There is implementation divergence.

See also LLVM issue 54881.

Proposed resolution (approved by CWG 2022-06-17):

(updated according to 2022-05-20, 2022-06-03, and 2022-06-17 CWG guidance)

Change in 9.5.4 [dcl.fct.def.coroutine] paragraph 9 as follows:

... The allocation function's name is looked up by searching for it in the scope of the promise type.