cmd/compile: cannot import "package" (type parameter bound more than once)

[csgura]

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

$ go version
go version devel go1.18-a412b5f Wed Dec 1 06:03:27 2021 +0000 darwin/amd64

Does this issue reproduce with the latest release?

yes

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

The Gotip Playground

What did you do?

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

What did you expect to see?

Compiled without error

What did you see instead?

./prog.go:5:2: could not import tptest/option (cannot import "tptest/option" (type parameter bound more than once), possibly version skew - reinstall package)

[danscales]

I'm looking at this, since it may relate to confusion with import/export, but @findleyr , you may also want to take a look when you get a chance, since the error happens in types2. Seems like we may somehow confused distinct type parameters during export/import across multiple packages. Problem goes away if the type params in definition of Ap2 are renamed (e.g. to A2/B2).

@griesemer

[findleyr]

Thanks for the repro. This should be a release blocker: there is something wrong with our type-parameter scoping.

[findleyr]

the error happens in types2

@danscales the error is coming from types2, but the bug must be in the importer. Type parameters are local to their function/type, and so must be bound only once.

[danscales]

The bug relates to the type parameter B being specified in Ap1, but not actually being used in Ap1 (applies to Ap2 as well). Because B is not used, we are somehow referencing the wrong B during definition and then export. Doesn't always cause a visible problem, but sometimes does (as in this issue).

[gopherbot]

Change https://golang.org/cl/368454 mentions this issue: cmd/compile: fix case where g.curDecl should be saved/restored