cmd/compile: failed to compile some recursive generic type

[johejo]

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

$ go version
go version devel go1.20-a719a78c1b Fri Aug 19 00:29:18 2022 +0000 darwin/arm64

Does this issue reproduce with the latest release?

tip only

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

go env Output

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/mitsuoheijo/Library/Caches/go-build"
GOENV="/Users/mitsuoheijo/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/mitsuoheijo/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/mitsuoheijo/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/mitsuoheijo/ghq/github.com/golang/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/mitsuoheijo/ghq/github.com/golang/go/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="devel go1.20-a719a78c1b Fri Aug 19 00:29:18 2022 +0000"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/mitsuoheijo/ghq/github.com/johejo/sandbox/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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bq/s3vkkxm17jq79lx5qy3856zh0000gn/T/go-build3085638013=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

type node[T any] struct {
	items    items[T]
	children items[*node[T]]
}

func (n *node[T]) f(i int, j int) bool {
	if len(n.children[i].items) < j {
		return false
	}
	return true
}

type items[T any] []T

func main() {
	_ = node[int]{}
	println("hello")
}

What did you expect to see?

$ go run .
hello

What did you see instead?

$ gotip run .
# github.com/johejo/sandbox
./main.go:9:22: n.children[i].items undefined (type go.shape.*uint8 has no field or method items)

This might be some regression.
I found this problem while using google/btree.
https://github.com/google/btree

[cuonglm]

It run ok on tip now: ee833ed

[hopehook]

I can reproduce this problem too.

go version devel go1.20-ee833ed Fri Aug 19 01:34:22 2022 +0000 linux/amd64

# command-line-arguments
./main.go:9:22: n.children[i].items undefined (type go.shape.*uint8 has no field or method items)

[cuonglm]

@hopehook Oh right, I messed up my operation.

It runs ok in nounified:

GOEXPERIMENT=nounified go run r.go

cc @mdempsky

[mdempsky]

Thanks for the report!

Further minimized test case:

package main

type s[T any] struct{ f T }

func f[T any]() {
	var x s[*struct{ g T }]
	_ = x.f.g
}

func main() {
	_ = f[int]
}

This is an issue with pointer shaping. The type items[*struct{g T}] gets pointer shaped to items[*byte], which means x.f has shape type *byte. We need to insert an implicit conversion back to *struct{g T} so that we can select the g field.

[hopehook]

FYI: bisect shows bad at this CL: https://go-review.googlesource.com/c/go/+/422235.

[mdempsky]

@hopehook Thanks, that's not surprising. That CL simply switches the default from GOEXPERIMENT=nounified to GOEXPERIMENT=unified, and it was already reported above that the issue goes away when setting GOEXPERIMENT=nounified.

FWIW, more useful would be if you can force GOEXPERIMENT=unified while bisecting. But I'm pretty sure that will bisect to CL 424734.

[hopehook]

@mdempsky Yes, you are right.

[gopherbot]

Change https://go.dev/cl/424936 mentions this issue: cmd/compile: fix unified IR regressions