go/types: 'under' panics on Alias type

[varungandhi-src]

Go version

go version go1.23.0 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/varun/Library/Caches/go-build'
GOENV='/Users/varun/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/varun/.asdf/installs/golang/1.23.0/packages/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/varun/.asdf/installs/golang/1.23.0/packages'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/varun/.asdf/installs/golang/1.23.0/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/varun/.asdf/installs/golang/1.23.0/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23.0'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/varun/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/varun/Code/play/go/underpanic/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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/tr/gylnxkts3bn0gyhhlrv8z9_m0000gn/T/go-build2694439591=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

Minimal example: https://github.com/varungandhi-src/underpanic

I had some code using using APIs like String on *types.Named, as well as functions like types.NewMethodSet.

I upgraded that from Go 1.22.4 to Go 1.23.0

What did you see happen?

go/types.(*Named).under(0x140001a6850)
        /Users/varun/.asdf/installs/golang/1.23.0/go/src/go/types/named.go:548 +0x38c
go/types.under({0x1007ca020, 0x140001a6850})
        /Users/varun/.asdf/installs/golang/1.23.0/go/src/go/types/under.go:16 +0x2c
go/types.writeObject(0x14000467890, {0x1007cdb38, 0x140001a4780}, 0x0)
        /Users/varun/.asdf/installs/golang/1.23.0/go/src/go/types/object.go:576 +0x734
go/types.ObjectString({0x1007cdb38, 0x140001a4780}, 0x0)
        /Users/varun/.asdf/installs/golang/1.23.0/go/src/go/types/object.go:613 +0x44
go/types.(*TypeName).String(...)
        /Users/varun/.asdf/installs/golang/1.23.0/go/src/go/types/object.go:619
main.main()
        /Users/varun/Code/play/go/underpanic/main.go:33 +0x164

What did you expect to see?

Functions like NewMethodSet and methods like String should continue to work in the presence of CGo.

[gabyhelp]

Related Issues and Documentation

• cmd/compile/internal/types2, go/types: don't panic when calling under/underlying #65344
• go/internal/gcimporter: untyped interface type parameter causes "panic: nil underlying" during testing #57015 (closed)
• cmd/go2go: panic with an embedded type param #43759
• cmd/compile: ICE compiling rangefunc for < 1.23 in combination with Cgo #67497 (closed)
• cmd/compile/internal/types2: unify.go:449: assertion failed, panic #67872 (closed)
• cmd/go: bootstrap with Go 1.21.x fail at tip, panic: godebug: Value of name not listed in godebugs.All: gotypesalias #64106 (closed)
• go/types, types2: stack overflow results after naming type parameter after interface name. #50517 (closed)
• cmd/compile: panic when using typeparams #46353 (closed)
• x/tools/go/packages: panic loading with LoadTypes #31163 (closed)
• cmd/cgo: go 1.3 panics on recursive use of typedef #8368 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[ianlancetaylor]

CC @griesemer @findleyr

[adonovan]

Many thanks for the very helpful bug report. The input program that reproduces the problem can be reduced to the following, which does not use cgo:

package main

type (
	T struct{}
	U = T
	S U
)

Loading it using this reduced main program:

package main

import (
	"go/types"
	"log"

	"golang.org/x/tools/go/packages"
)

func main() {
	cfg := &packages.Config{Mode: packages.NeedTypes}
	pkgs, err := packages.Load(cfg, ".")
	if err != nil {
		log.Fatal(err)
	}
	obj := pkgs[0].Types.Scope().Lookup("S").(*types.TypeName)
	_ = obj.String() // panics
}

produces the same crash:

processing type: type golang.org/x/tools/issue68877.T struct{}
panic: Named.check == nil but type is incomplete

goroutine 1 [running]:
go/types.(*Named).under(0x140000c0b60)
	/Users/adonovan/w/goroot/src/go/types/named.go:548 +0x38c
go/types.under({0x1029e9bc0, 0x140000c0b60})
	/Users/adonovan/w/goroot/src/go/types/under.go:16 +0x2c
go/types.writeObject(0x140002910b0, {0x1029ed7d8, 0x140005304b0}, 0x0)
	/Users/adonovan/w/goroot/src/go/types/object.go:576 +0x734
go/types.ObjectString({0x1029ed7d8, 0x140005304b0}, 0x0)
	/Users/adonovan/w/goroot/src/go/types/object.go:613 +0x44
go/types.(*TypeName).String(...)
	/Users/adonovan/w/goroot/src/go/types/object.go:619
main.main()
	/Users/adonovan/w/xtools/issue68877/main.go:33 +0x164
exit status 2

[adonovan]

Reducing again to eliminate packages.Load:

func TestIssue68877(t *testing.T) {
	setGotypesalias(t, true)
	const src = `package main

type (
	T struct{}
	U = T
	S U
)`

	pkg := mustTypecheck(src, nil, nil)
	S := pkg.Scope().Lookup("S").(*TypeName)
	_ = S.String()
}

[griesemer]

cc: @timothy-king for visibility

[griesemer]

Analysis so far: The underlying problem (no pun intended) is that Named.Underlying may end up calling Unalias which may return a Named type which is not the actual underlying type. We can follow that chain to the end which addresses this particular issue but causes other tests to loop endlessly, for not yet fully understood reasons.

The endless seem to be caused by invalid code (alias cycles). Should have a tentative CL later today.

[gopherbot]

Change https://go.dev/cl/605757 mentions this issue: go/types, types2: fix Named.Underlying in the presence of Alias types

[griesemer]

Gopherbot, please create a backport issue for Go 1.23.

[callthingsoff]

Gopherbot, please create a backport issue for Go 1.23.

We need ‘@‘ the bot.

[findleyr]

@gopherbot, please create a backport issue for Go 1.23: this is a regression in the type checker when run with GODEBUG=gotypesalias=1, the new default.

[gopherbot]

Backport issue(s) opened: #68894 (for 1.23).

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

[gopherbot]

Change https://go.dev/cl/605977 mentions this issue: go/types, types2: Named.cleanup must also handle *Alias types