cmd/compile: handle alias types in recursive types more consistently

[griesemer]

The following code

package p

var x T[B]

type T[_ any] struct{}
type A T[B /* ERROR invalid use of type alias */ ]
type B = T[A]

reports an error when we use B in a recursive type definition due to the way type aliases are handled inside the type checker. In this (and possibly many other similar) cases, the error can be avoided by restructuring the code slightly. This version compiles without error (declaration of x moved past the type declarations):

package p

type T[_ any] struct{}
type A T[B]
type B = T[A]

var x T[B]

The compiler should be able to avoid this error on its own.

Root cause: Currently, the type checker doesn't have a "forwarding mechanism" for type aliases that are being "used" before their respective type is fully known. To solve this problem in general, a forwarding mechanism/type needs to be introduced. This is only an issue with some recursive type definitions involving type aliases.

cc: @findleyr

[griesemer]

See #50259 and #50276.

[gopherbot]

Change https://golang.org/cl/379916 mentions this issue: cmd/compile/internal/types2: report an error when using a broken alias

[gopherbot]

Change https://golang.org/cl/380056 mentions this issue: cmd/compile/internal/types2: reorder object processing to avoid broken aliases

[griesemer]

This error doesn't appear anymore due to some of the (partial) fixes done with the CLs above. There's still more to do to handle aliases correctly in all cases but that's separate from this issue. We should add a test to cover this specific case and then close this issue.

[gopherbot]

Change https://go.dev/cl/412235 mentions this issue: go/types, types2: add test case for issue for coverage