cmd/compile: ICE on method value involving imported anonymous interface

[zapo]

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

$ go version
go version go1.20.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
10:52 $ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/zapo/.cache/go-build"
GOENV="/home/zapo/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/zapo/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/zapo/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/zapo/.asdf/installs/golang/1.20.1/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/zapo/.asdf/installs/golang/1.20.1/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/zapo/src/test/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2227726188=/tmp/go-build -gno-record-gcc-switches"

What did you do?

./pkg/pkg.go

package pkg

func Start() interface{ Stop() } {
	return new(Stopper)
}

type Stopper struct{}

func (s *Stopper) Stop() {}

./main.go

package main

import "test/pkg"

func main() {
	stop := start()
	defer stop()
}

func start() func() {
	return pkg.Start().Stop
}

What did you expect to see?

Successful build.

What did you see instead?

./pkg/pkg.go:3:6: internal compiler error: assertion failed

I can fix it by allocating the return of Start() or using a named interface, or avoiding to return the pointer to Stop interface by chaning start() to:

func start() func() {
	return func() { pkg.Start().Stop() }
}

Also I couldn't reproduce it if merging in the same package.

[thanm]

This is also reproducible on tip.

@golang/compiler @mdempsky

[cuonglm]

@mdempsky It seems to me that the assertion failed due to mismatch between method selection position. There're two distinct anonymous interface types interface { Stop() } on line 3 and line 4, the corresponding Stop method also got different position. The wrapperFn uses the one on line 3, but the pkg.Start() use the one on line 4.

[mdempsky]

@cuonglm Yeah, I think the assertion can just be relaxed. I have a CL, just power was out in my apt all day.

[gopherbot]

Change https://go.dev/cl/471676 mentions this issue: cmd/compile: relax overly strict assertion

[mdempsky]

@gopherbot Please backport to 1.20.

[gopherbot]

Backport issue(s) opened: #58776 (for 1.20).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.