cmd/go2go: recursive type constraint fails with confusing error

[benjaminjkraft]

Here's a type:

type Rec[type T Rec[T]] interface{ r(T) }

Currently go2go complains with confusing errors; the first declaration fails with Rec is not a generic type (it obviously is) and the equivalent type Rec[type T Rec] interface{ r(T) } fails with Rec is not an interface (it also obviously is).

I assume this shouldn't be allowed: to know if Rec[T] is a valid type for T to implement, we need to know if T implements Rec[T], which is circular. But I can't find anything in the draft design that would prohibit it.

(playground for convenience)

[ianlancetaylor]

I don't think these cases should be permitted but it does seem that the errors generated by the type checker are not ideal.