runtime: in generic code, convert iface to type failed with "types from different scopes"

[rguilmont]

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

go version go1.18beta2 linux/amd64

Does this issue reproduce with the latest release?

yes

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

go env

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/romain/.cache/go-build"
GOENV="/home/romain/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/romain/go/pkg/mod"
GONOPROXY="XXXXXXXXXXXXXXXXXXX"
GONOSUMDB="XXXXXXXXXXXXXXXXXXX"
GOOS="linux"
GOPATH="/home/romain/go"
GOPRIVATE="XXXXXXXXXXXXXXXXX"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/romain/sdk/go1.18beta2"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/romain/sdk/go1.18beta2/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18beta2"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/romain/Documents/git/github/replicate-issue/types-from-different-scopes-issue/go.mod"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build838965849=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Using generics and interface conversion, i get the following error at runtime :

panic: interface conversion: either.Either[go.shape.string_0,go.shape.interface {}_1] is either.right[github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga.Test,interface {}], not either.right[github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga.Test,interface {}] (types from different scopes)

goroutine 1 [running]:
github.com/rguilmont/types-from-different-scopes-issue/pkg/either.Fold[...]({0x4b2c00, 0xc00009e210}, 0x49c850, 0x49c858)
        /home/romain/Documents/git/github/replicate-issue/types-from-different-scopes-issue/pkg/either/either.go:42 +0xc5
main.main()
        /home/romain/Documents/git/github/replicate-issue/types-from-different-scopes-issue/pkg/cmd/main.go:13 +0x65

To check the code, here it is with instruction to replicate : https://github.com/rguilmont/types-from-different-scopes-issue

What did you expect to see?

It works! test

Thanks !

[dmitshur]

Thanks for the report. I don't know generics to say if this is expected or not, but I checked and this behavior reproduces with go1.18rc1 as well.

CC @findleyr, @mdempsky.

[mdempsky]

At the very least, I don't think "go.shape" should appear in any user visible error messages? I expect the panic message should mention the specific types instead.

/cc @danscales @randall77

[danscales]

In panicdottypeE, the have and want type descriptors look completely identical (I'm assuming the gcdata is identical too, didn't try to look past the first byte in gdb yet):

(dlv) p have
*runtime._type {size: 16, ptrdata: 8, hash: 2221142398, tflag: tflagUncommon|tflagExtraStar|tflagNamed (7), align: 8, fieldAlign: 8, kind: 25, equal: runtime.strequal, gcdata: *1, str: 41281, ptrToThis: 48864}
(dlv) p want
*runtime._type {size: 16, ptrdata: 8, hash: 2221142398, tflag: tflagUncommon|tflagExtraStar|tflagNamed (7), align: 8, fieldAlign: 8, kind: 25, equal: runtime.strequal, gcdata: *1, str: 41281, ptrToThis: 48864}

Seems like we maybe ended up with identical run-time types that should have been created as the same type from the start or deduplicated. The "types from different scopes" message is really just saying that's the guess, given that there is no other reason why the types aren't the same type (the other possibility printed is that they are in different packages).

[rfindley]

@dmitshur I think you tagged me by accident, maybe intending findleyr instead. No worries, but thought you might want to know so you can re-tag.

[dmitshur]

@rfindley You're right, thanks. Sorry for the noise! I edited my earlier comment now to CC the intended person.

[rguilmont]

In panicdottypeE, the have and want type descriptors look completely identical (I'm assuming the gcdata is identical too, didn't try to look past the first byte in gdb yet):

(dlv) p have
*runtime._type {size: 16, ptrdata: 8, hash: 2221142398, tflag: tflagUncommon|tflagExtraStar|tflagNamed (7), align: 8, fieldAlign: 8, kind: 25, equal: runtime.strequal, gcdata: *1, str: 41281, ptrToThis: 48864}
(dlv) p want
*runtime._type {size: 16, ptrdata: 8, hash: 2221142398, tflag: tflagUncommon|tflagExtraStar|tflagNamed (7), align: 8, fieldAlign: 8, kind: 25, equal: runtime.strequal, gcdata: *1, str: 41281, ptrToThis: 48864}

Seems like we maybe ended up with identical run-time types that should have been created as the same type from the start or deduplicated. The "types from different scopes" message is really just saying that's the guess, given that there is no other reason why the types aren't the same type (the other possibility printed is that they are in different packages).

From what i understand, and the "bypass" branch i've put to my repo, it seems that in a way ( i may be completely wrong, i don't have your understanding of the go runtime 😁 ) the error message is relevant : (types from different scopes) :

either.right[github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga.Test,interface {}], not either.right[github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga.Test,interface {}]

in the first case, the type was created inside the package ( namely github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga ) and i tried to "cast" it with a type from an imported package ( same, github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga ).

Once i move the type in another package, the issue disappear ( as you can see in the bypass branch ) because then in each package i work with types imported from another package. The issue looks related to scopes, maybe there's something lost in some way that prevent the casting though it should be valid.

I hope i was clear enough and i can help 🥲

[randall77]

Looks like we have two different itabs representing the same type/interface combo.

The itabs are almost identical. They have the same name according to the linker, and all their fields are the same except the hash field. Because the has field is different, the linker doesn't deduplicate like we would normally expect.

The types have different hashes, because one time they are computed as the hash of:

either.right[either.Test,interface {}]

or

either.right[github.com/rguilmont/types-from-different-scopes-issue/pkg/pkga.Test,interface {}]

I think the latter is the right thing to be hashing. Not sure how we get to the former. I think it has something to do with the LocalPkg being the package being compiled, not necessarily the package where the named type originates.

[randall77]

Hm, looks like when compiling pkga, we make a symbol right["".Test,interface {}] in package either. But the "" in that symbol refers to pkga. But right is in package either, so we use either to substitute into the symbol name, which is not always right.

[gopherbot]

Change https://go.dev/cl/387974 mentions this issue: cmd/compile: use package path instead of "" inside instantiation of nonlocal type

[cagedmantis]

What is the status of the CL linked to this issue? It is a release blocker and we are working our way to a release.

[randall77]

There is a CL that fixes this issue. I'm investigating a possibly better fix, should know one way or another by EOD.