go/types, types2: alias types are restricted to basic interfaces, should allow arbitrary interfaces

[zigo101]

What version of Go are you using (go version)?

$ go version
go version go1.18rc1 linux/amd64

What did you do?

type C[T any] interface{~int; M() T}
type C1 = C[bool] // error
type C2 = comparable // error
type C3 = interface {~[]byte | ~string} // error

I didn't find this is documented in tip spec.

[ianlancetaylor]

CC @griesemer

[griesemer]

This is just a bug. I don't see a reason why there should be a restriction here. It's a trivial change but too late for 1.18.

[gopherbot]

Change https://go.dev/cl/392715 mentions this issue: go/types, types2: alias type declarations may refer to constraint types