CWG3193 [dcl.friend] Missing rule banning friend declarations outside classes

[timsong-cpp]

Full name of submitter (unless configured in github; will be published with the issue):

Reference (section label): [dcl.friend]

Link to reflector thread (if any): see CWG mattermost

Issue description:

We seem to be missing a rule that actually disallows friend declarations outside classes. At least, I haven't been able to locate one after a quick look at all the use of friend in [dcl] and [class].

Suggested resolution:

[t3nsor]

Some other issues with friendship that should perhaps be folded into this issue:

• There appears to be no normative text that specifies which class grants friendship (i.e. the class whose scope is inhabited by the friend declaration).
• We say that a friend-type-declaration declares the designated type as a friend, but we don't specify the effect of the friend specifier on a function declaration.

[jensmaurer]

CWG3193