types2, go/types: self-referential type parameter list is not accepted

[griesemer]

Example:

package p
type Eq[a Eq[a]] interface {
	Equal(that a) bool
}

produces

:2:13: Eq is not a generic type

Should this be permitted?

cc: @ianlancetaylor

[griesemer]

In any case, the error message needs to be fixed. Eq is a generic type.

[ianlancetaylor]

I can't think of a reason to forbid this. It seems well-defined. It would be more problematic in a function definition, where we would have to derive an operation set, but of course this can't happen in a function definition and the problem doesn't arise in a type definition.

[gopherbot]

Change https://golang.org/cl/348090 mentions this issue: cmd/compile/internal/types2: handle recursive type parameter constraints