spec: disallow recursive embedded interfaces

[griesemer]

The following declaration

type E interface {
    E
}

is accepted by neither the gc collection of compilers nor gccgo (interface type loop
involving E).

Arguably it is a legal declaration; at least there is no wording in the spec against it.

This is either a) a compiler bug, or b) the spec needs to be clearer. Also, this is a
completely academic issue with no practical implications...

[griesemer]

Comment 1:

On a related subject, the spec does not say anything about struct types referring to
themselves directly or indirectly via field types (of named or anonymus fields).
With respect to the above example: While this special case may be easy to detect, more
complicated indirect scenarios are possible:
type E1 interface { E2 }
type E2 interface { E3 }
...
type En interface { E1 }
all of which are empty and thus theoretically legal. This will be harder to detect and
has no practical value. Such self-recursive interface types should not be allowed as per
current compiler implementation.
Changing this to a documentation issue: The spec should probably clarify what forms of
direct and indirect recursions are legal in type declarations.

Labels changed: added documentation, removed compilerbug, languagechange.

[griesemer]

Comment 2:

Issue #1815 has been merged into this issue.

[rsc]

Comment 3:

I don't believe that
type E interface {
    E
}
is any more legal than
type T struct {
    T
}
In both cases there exists a minimal fixed point
that could in theory be arrived upon, but nothing
else in the language requires such analysis.
It's not just this specific case either.  I think the
interface embedding graph needs to be cycle free.
Russ

[griesemer]

Comment 4:

I think we all agree.
- gri

[rsc]

Comment 6:

Labels changed: added priority-someday.

[rsc]

Comment 7:

Labels changed: added priority-go1.

[robpike]

Comment 8:

Owner changed to builder@golang.org.

[rsc]

Comment 10:

Labels changed: added go1-must.

[rsc]

Comment 11:

Status changed to Started.

[rsc]

Comment 12:

This issue was closed by revision 388816a.

Status changed to Fixed.