go/types: problem with alias type

[griesemer]

The following code

package p

type F = func(T)

type T interface {
	m(F)
}

type t struct{}

func (t) m(F) {}

var _ T = &t{}

doesn't type-check:

y.go:13:11: cannot use &(t literal) (value of type *t) as T value in variable declaration: wrong type for method m

but the code is correct (replacing F with func(T) everywhere works).

[gopherbot]

Change https://golang.org/cl/147286 mentions this issue: cmd/compile: reintroduce work-around for cyclic alias declarations

[griesemer]

Simpler reproducer (underlying cause is the same). This code should not be accepted but is:

package p

type F = func(T)

type T interface {
	m(F) int
}

var _ = T(nil).m("foo")

yet, this code (declaration of F and T swapped) is correctly not accepted:

package p

type T interface {
	m(F) int
}

type F = func(T)

var _ = T(nil).m("foo")

[griesemer]

This is essentially the same problem as #25838 in the compiler: An alias declaration doesn't use a specific Alias type representation (or "forwarder") and is initialized to Typ[Undefined] before it is computed but then accessed before the computed value is stored.

A simple work-around might be the same as we do in the compiler which is to type-check non-alias type declarations first.

[griesemer]

The work-around does address the issue at hand, but it's easy to create slightly more complex cases where it won't work again, even with the work-around:

package p

// Without this type declaration the go/types will correctly report an error
// for the variable declaration below; with it it will accept this code silently
// and incorrectly.
type S F

type F = func(T)

type T interface {
	m(F) int
}

var _ = T(nil).m("foo")

[gopherbot]

Change https://golang.org/cl/147597 mentions this issue: go/types: avoid certain problems with recursive alias type declarations

[gopherbot]

Change https://golang.org/cl/151339 mentions this issue: [release-branch.go1.11] cmd/compile: reintroduce work-around for cyclic alias declarations

[griesemer]

@gopherbot please file this (partial fix) for backport against 1.11. This is a regression.

[gopherbot]

Backport issue(s) opened: #28972 (for 1.11).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://golang.org/wiki/MinorReleases.