go/types, types2: confusing error message when using comparable in a union

[Code-Hex]

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

$ go version
go version devel go1.18-c2397905e0 Sat Nov 13 03:33:55 2021 +0000 darwin/arm64

Does this issue reproduce with the latest release?

No, because it includes generics

What operating system and processor architecture are you using (go env)?

Whatever the go playground runs on

What did you do?

https://gotipplay.golang.org/p/FSMOjCXAtSL

What did you expect to see?

can compile

What did you see instead?

compile error

./prog.go:6:66: cannot use comparable in union (interface contains methods)

[syumai]

I got the same error in a similar case.
https://gotipplay.golang.org/p/y_eVVr0rp7H

comparable interface should be treated as a union of enumerated comparable types, so expect to be able to use it in a union. (I think comparable interface does not contain method as a constraint.)
If it is not allowed to use comparable within a union, then I want a different error message like below.

cannot use comparable in union (comparable can not be a union element)

[findleyr]

CC @griesemer

This is working as intended, at least for Go 1.18. See #45346 (comment) for more context. With respect to computing type sets, comparable behaves like an interface with methods.

I agree that we should improve the error message, however.

[Code-Hex]

Thank you for your comment.

To be honest, I didn't understand why comparable types are not allowed to be used in unions. (As spec)
https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#comparable-types-in-constraints

This is just curious. Are there any resources or links that can help me?

[griesemer]

Eventually there will be better documentation. We're in the middle of writing it. Suffice to say that for 1.18 at least, comparable cannot be part of a multi-element union.

Will keep this open to improve the error message.

[gopherbot]

Change https://golang.org/cl/372674 mentions this issue: cmd/compile/internal/types2: better error message when using comparable in union

[gopherbot]

Change https://golang.org/cl/372694 mentions this issue: spec: clarify that comparable cannot be a union element

[gopherbot]

Change https://golang.org/cl/372954 mentions this issue: go/types: better error message when using comparable in union