cmd/go: compiler panic on recursive structures

[sisoftrg]

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

$ go version
go version go1.16.3 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
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.3"
GCCGO="/usr/bin/gccgo"
AR="ar"
CC="x86_64-pc-linux-gnu-gcc"
CXX="x86_64-pc-linux-gnu-g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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-build3022878780=/tmp/go-build -gno-record-gcc-switches"

What did you do?

In case of recursive structures (type X includes Y and type Y includes X) by mistake I got panic in compiler instead of readable error. Minimal example to reproduce:
https://play.golang.org/p/pvO3GSWnExA
So simple in this example, but hard to find such issue origin in big project without descriptive error.

What did you expect to see?

./1.go:7:6: invalid recursive type X
        /home/1.go:7:6: X refers to
        /home/1.go:12:6: Y refers to
        /home/1.go:7:6: X

What did you see instead?

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0xa43ea5]

goroutine 1 [running]:
cmd/compile/internal/gc.findTypeLoop(0xc0000d2120, 0xc000110788, 0xc00009c240)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:202 +0x105
cmd/compile/internal/gc.findTypeLoop(0xc0000d2060, 0xc000110788, 0x1)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:216 +0x2c5
cmd/compile/internal/gc.findTypeLoop(0xc0000d2000, 0xc000110788, 0xc000098110)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:202 +0x117
cmd/compile/internal/gc.findTypeLoop(0xc0000d8000, 0xc000110788, 0x0)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:216 +0x2c5
cmd/compile/internal/gc.findTypeLoop(0xc0000d3f80, 0xc000110788, 0x2)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:202 +0x117
cmd/compile/internal/gc.reportTypeLoop(0xc0000d3f80)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:240 +0x76
cmd/compile/internal/gc.dowidth(0xc0000d3f80)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:282 +0x1105
cmd/compile/internal/gc.widstruct(0xc0000d2000, 0xc0000d2000, 0x0, 0x1, 0x8)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc0000d2000)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.widstruct(0xc0000d3f80, 0xc0000d3f80, 0x0, 0x1, 0x18)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc0000d3f80)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.widstruct(0xc0000d2060, 0xc0000d2060, 0x0, 0x1, 0x8)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc0000d2060)
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.resumecheckwidth()
        /usr/lib/go/src/cmd/compile/internal/gc/align.go:526 +0x4c
cmd/compile/internal/gc.typecheckdef(0xc0000b9950)
        /usr/lib/go/src/cmd/compile/internal/gc/typecheck.go:3695 +0x8e5
cmd/compile/internal/gc.typecheck1(0xc0000b9950, 0x4, 0x0)
        /usr/lib/go/src/cmd/compile/internal/gc/typecheck.go:343 +0xbace
cmd/compile/internal/gc.typecheck(0xc0000b9950, 0x4, 0x0)
        /usr/lib/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.typecheck1(0xc0000c8280, 0x1, 0x0)
        /usr/lib/go/src/cmd/compile/internal/gc/typecheck.go:2077 +0x4e65
cmd/compile/internal/gc.typecheck(0xc0000c8280, 0x1, 0x0)
        /usr/lib/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.Main(0xcc8d40)
        /usr/lib/go/src/cmd/compile/internal/gc/main.go:603 +0x2aa5
main.main()
        /usr/lib/go/src/cmd/compile/main.go:52 +0xb1

[cuonglm]

Duplicated of #44266

[ALTree]

Thanks for reporting this.

As @cuonglm pointed out, this is #44266, and I've confirmed it's already fixed on tip. Closing here since no further action is required.