CWG2588 friend declarations and module linkage

[jensmaurer]

The linkage of friend functions and classes is unclear, if the first (and possibly only) declaration is in a class.

https://cplusplus.github.io/CWG/issues/2588.html

[erichkeane]

EWG Discussed this during the June 9th, 2022 EWG telecon, and the following polls were taken:

A friend's linkage should be affected by the linkage of the befriending class (#1).

SF	F	N	A	SA
0	1	0	4	0

Result: Not Consensus

A friend's linkage should be affected by the presence/absence of export on the containing class definition itself (#2).

SF	F	N	A	SA
0	1	4	0	0

Result: Not Consensus

A friend's linkage should be affected by the presence/absence of export on the containing class definition itself, but ONLY if the friend is a definition (#2, modified by Jason's suggestion).

SF	F	N	A	SA
3	3	0	0	1

Result: Consensus

A friend's linkage should be determined per 11.8.4p4 alone (#3).

SF	F	N	A	SA
0	0	1	4	0

Result: Not Consensus

A friend's linkage should be as-if the declaration inhabited the nearest enclosing namespace scope (#4).

SF	F	N	A	SA
0	0	4	0	1

Result: Not Consensus

Confirmation of the 3rd poll is required to go through EWG Electronic polling.

[jfbastien]

This was voted on in the June 2022 EWG polls. Results are here.

🗳 Poll: A friend’s linkage should be affected by the presence/absence of export on the containing class definition itself, but ONLY if the friend is a definition (option #2, modified by Jason’s suggestion). This resolves CWG2588.

Note: Jason’s suggestion: A friend declaration shouldn’t affect linkage if it is not a definition. Exporting definition of class exports "friendliness" but not export of friend function itself.

SF	F	N	A	SA
4	12	4	1	1
Abstain: 6

Poll outcome: ✅ consensus