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cmd/compile/internal/types2: predeclared type "comparable" must not be visible before Go 1.18 #46090

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mdempsky opened this issue May 10, 2021 · 6 comments
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cmd/compile/internal/types2: predeclared type "comparable" must not be visible before Go 1.18 #46090

mdempsky opened this issue May 10, 2021 · 6 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Go1.18

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@mdempsky
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This package produces an "undefined: any" error when compiled with go tool compile -G=1:

package p
type _ any

but this package successfully type-checks (but fails later anyway when compiled with -G=3):

package p
type _ comparable

They both produce errors on go2goplay.golang.org.

/cc @griesemer @findleyr
@mdempsky mdempsky added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label May 10, 2021
@mdempsky mdempsky added this to the Go1.18 milestone May 10, 2021
@griesemer griesemer self-assigned this May 11, 2021
@findleyr findleyr self-assigned this May 13, 2021
@findleyr findleyr mentioned this issue May 30, 2021
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@griesemer
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griesemer commented Jun 29, 2021 •
edited

I believe this is working as intended: any may only be used in constraint position (for now, eventually we may make it generally acceptable in place of interface{}). comparable is a predeclared interface type that may be used in types (incl. type declarations); but using a type that is (or embeds) comparable as type for an ordinary variable, or field results in an error.

[edit]: But neither any nor comparable should be accessible for language versions before Go 1.18 (any won't be accessible because one cannot write a constraint before Go 1.18).

@mdempsky
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I think I'm inclined to agree. I'm not sure why I thought type _ comparable should have been invalid, except that go2go rejected it and I must have assumed go/types was actually rejecting it.

Looking more closely at the go2go error diagnostics, I suspect the error message is actually because go2go is generating invalid code in this case. (undefined: comparable looks like a cmd/compile error message, not a go/types one.)

@griesemer griesemer changed the title cmd/compile/internal/types2: "type _ any" rejected but "type _ comparable" accepted cmd/compile/internal/types2: predeclared type "comparable" must not be visible before Go 1.18 Jun 29, 2021
@gopherbot
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Change https://golang.org/cl/331517 mentions this issue: [dev.typeparams] cmd/compile/internal/types2: "comparable" must not be visible before Go 1.18

@ianlancetaylor
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In case it helps, go2go reports go/types errors with a prefix of "type checking failed for PACKAGE". If you see that message, there is probably a problem with the code being translated. If you don't see that message, it's probably a bug in go2go.

@mdempsky
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@ianlancetaylor Thanks for the tip.

gopherbot pushed a commit that referenced this issue Jul 1, 2021
@griesemer
[dev.typeparams] cmd/compile/internal/types2: "comparable" must not b…
1cd505c 
…e visible before Go 1.18

While at it, clean up the setup of comparable in universe.go.

Fixes #46090

Change-Id: I9655b3e137a03763d677d9a2a730c5570ccff6dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/331517
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
@griesemer
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This was fixed by

https://golang.org/cl/331517 in types2
https://golang.org/cl/335032 in go/types

@golang golang locked and limited conversation to collaborators Jun 23, 2023
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