go/types, types2: type-checker internal type printing doesn't print embedded fields that are alias types correctly

[griesemer]

Given

type E = map[string]int
type S struct {
   E
}

the struct S will be printed with an embedded field named map[string]int because we have lost information about its alias name.

This affects debugging and test cases, e.g., src/go/internal/gccgoimporter/importer_test.go:97 wants "type S struct{b int; A2" but we only have "type S struct{b int; map[Y]Z}".

The relevant printing code is in go/types/typestring.go, function writeType.

[griesemer]

If we solve the type alias related issues (by introducing an explicit type alias node), this issue would probably be fixed as well.

[gopherbot]

Change https://golang.org/cl/293962 mentions this issue: [dev.typeparams] go/types, types2: revert fancy struct printing (fixes x/tools tests)

[gopherbot]

Change https://golang.org/cl/348737 mentions this issue: cmd/compile/internal/types2: print correct embedded field name

[ianlancetaylor]

@griesemer This is in the 1.18 milestone; time to move to 1.19? Thanks.

[griesemer]

CL 381776, if it can be made to work, would fix this. Leaving open for a bit longer. But fine to move to 1.19 once we decide to move forward with the release.

[gopherbot]

Change https://golang.org/cl/381958 mentions this issue: cmd/compile/internal/types2: correctly print embedded struct fields that are aliases

[griesemer]

There's no easy fix that doesn't require major restructuring. We have lived with this for a long time, so not a regression. Leaving for 1.19.

[mvdan]

If we solve the type alias related issues (by introducing an explicit type alias node), this issue would probably be fixed as well.

I would absolutely love having recorded information about aliases in go/types. This information is currently missing, so I'm attempting to piece it back together for a tool of mine that needs to be aware of type aliases, because their name matters when they are used as an anonymous struct field. The code I came up with to do that reconstruction is complex, buggy, and very hard to debug.

I am more than happy to help any way I can, be it reviewing code, testing, etc. I would hope that the type alias information would be available to go/types users, perhaps via an extra field in the types.Info struct.

[griesemer]

A proper fix will require quite a bit of work on alias type representation. We won't get to this for 1.19, but we may want to make it a goal for 1.20. Adjusting milestone.