spec: Document rules for recursive type and other self-referential decls

[nico]

The spec says "An interface type T may not embed itself or any interface type that
embeds T, recursively.", but nothing like this is mentioned for structs.

(A struct containing itself would obviously need infty memory, but the spec should
probably still explicitly mention that these are illegal while a struct containing a
pointer to itself is fine

[robpike]

Comment 1:

It could embed a pointer to itself, though, and that works just fine. It's pointless to
embed a non-pointer recursively this way. Is that really worth explaining in the spec?

[griesemer]

Comment 2:

This applies also to named fields for structs, and arrays, too.
In general, all "composite value types" cannot contain themselves directly or indirectly
unless there's a reference type between.
It's somewhat obvious but it probably should be spec'ed out explicitly, after all it's
something that typecheckers go though some length to check.

Labels changed: added documentation.

Owner changed to @griesemer.

Status changed to Accepted.

[rsc]

Comment 3:

Labels changed: added go1.3maybe.

[rsc]

Comment 4:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 5:

Labels changed: added repo-main.