cmd/compile: import issue with self-referential generic interface type

[rogpeppe]

go version devel go1.18-b86e8dd0f3 Thu Sep 9 09:06:46 2021 +0000 linux/amd64

I see an unexpected error when I run go test on the following code:

-- p.go --
package p

type I[T I[T]] interface {
	F() T
}
-- p_test.go --
package p

import "testing"

func TestP(t *testing.T) {}

The error that I see is:

# github.com/rogpeppe/generic/set-bug
vet: ./p.go:3:10: I is not a generic type
# github.com/rogpeppe/generic/set-bug.test
/tmp/go-build1456973644/b001/_testmain.go:13:8: could not import github.com/rogpeppe/generic/set-bug (cannot import "github.com/rogpeppe/generic/set-bug" (type parameter bound more than once), possibly version skew - reinstall package)
FAIL	github.com/rogpeppe/generic/set-bug [build failed]

[jayconrod]

This appears to be a compiler bug. I was able to reproduce it using this build script, outside of the go command:

#!/bin/bash

set -eu

work=$(mktemp -d)
echo '$work = '$work >&2
src=$(pwd)
goroot=$(go env GOROOT)

mkdir "$work/b001"
cat >"$work/b001/importcfg" <<EOF
packagefile testing=$goroot/pkg/linux_amd64/testing.a
EOF
go tool compile -o "$work/b001/out.a" -p example.com/test -lang=go1.18 -complete -importcfg="$work/b001/importcfg" -pack "$src/p.go" "$src/p_test.go"

mkdir "$work/b002"
cat >"$work/b002/importcfg" <<EOF
packagefile os=$goroot/pkg/linux_amd64/os.a
packagefile testing=$goroot/pkg/linux_amd64/testing.a
packagefile testing/internal/testdeps=$goroot/pkg/linux_amd64/testing/internal/testdeps.a
packagefile example.com/test=$work/b001/out.a
EOF
go tool compile -o "$work/b002/out.a" -p main -lang=go1.18 -complete -importcfg="$work/b002/importcfg" -pack "$src/_testmain.go"

_testmain.go is generated:

// Code generated by 'go test'. DO NOT EDIT.

package main

import (
        "os"

        "testing"
        "testing/internal/testdeps"


        _test "example.com/test"



)

var tests = []testing.InternalTest{

        {"TestP", _test.TestP},

}

var benchmarks = []testing.InternalBenchmark{

}

var examples = []testing.InternalExample{

}

func init() {
        testdeps.ImportPath = "example.com/test"
}



func main() {

        m := testing.MainStart(testdeps.TestDeps{}, tests, benchmarks, examples)

        os.Exit(m.Run())

}

[cuonglm]

This seems to be the problem in the importer. After creating stub declaration, it recursively declaring the type, which in turn named.SetTypeParams(tparams) called twice for the same tparams.

Maybe we can just check if the name already exists and returns before setup stub declaration:

if r.currPkg.Scope().Lookup(name) != nil {
	return
}

cc @findleyr

[findleyr]

Thanks for the report. Somewhat related: #48098. We need more controls around recursive instantiation.

CC @griesemer

[cuonglm]

Thanks for the report. Somewhat related: #48098. We need more controls around recursive instantiation.

CC @griesemer

Note that the package build ok with cmd/compile, and typecheck ok with go/types.

The problem only happens when import package.

[findleyr]

The problem only happens when import package.

Ack, right. Perhaps your suggested fix is the way to proceed. I will look into it.

[gopherbot]

Change https://golang.org/cl/349009 mentions this issue: cmd/compile: prevent duplicated stub declaration in importReader.obj

[gopherbot]

Change https://golang.org/cl/349010 mentions this issue: go/internal/gcimporter: prevent duplicated stub declaration in importReader.obj