cmd/compile: panic if recursive type with field of error type

[bmizerany]

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

$ go version
go version go1.16.4 darwin/amd64

Does this issue reproduce with the latest release?

Yes. I have reproduced it with go1.16.7 darwin/amd64

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

go env Output

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/bmizerany/Library/Caches/go-build"
GOENV="/Users/bmizerany/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/bmizerany/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/bmizerany/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.7"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/bmizerany/go/src/resume/go.mod"
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 x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/rf/6fdgwbmj2rxf1jw13q4fv7h80000gn/T/go-build1864976941=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I wrote a little program with a recursive type which also contained a field of type error, and then ran both go test and go build:

$ cat hrm.go
package hrm

type T struct {
	E error
	T T
}

What did you expect to see?

A helpful error that I made a recursive type and the location where it was detected in the source.

What did you see instead?

A panic from cmd/compile:

hrm % go build           
# hrm
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x1641d8e]

goroutine 1 [running]:
cmd/compile/internal/gc.findTypeLoop(0xc000370240, 0xc0000ca8d8, 0xc00040a040)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:202 +0xce
cmd/compile/internal/gc.findTypeLoop(0xc000400fc0, 0xc0000ca8d8, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:216 +0x2c5
cmd/compile/internal/gc.findTypeLoop(0xc000400f60, 0xc0000ca8d8, 0x11)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:202 +0x117
cmd/compile/internal/gc.reportTypeLoop(0xc000400f60)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:240 +0x76
cmd/compile/internal/gc.dowidth(0xc000400f60)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:282 +0x1105
cmd/compile/internal/gc.widstruct(0xc000400f60, 0xc000400f60, 0x0, 0x1, 0x20)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc000400f60)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.widstruct(0xc000400fc0, 0xc000400fc0, 0x0, 0x1, 0x8)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:114 +0xd3
cmd/compile/internal/gc.dowidth(0xc000400fc0)
	/usr/local/go/src/cmd/compile/internal/gc/align.go:435 +0x4cf
cmd/compile/internal/gc.resumecheckwidth()
	/usr/local/go/src/cmd/compile/internal/gc/align.go:526 +0x4c
cmd/compile/internal/gc.typecheckdef(0xc000408000)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:3695 +0x8e5
cmd/compile/internal/gc.typecheck1(0xc000408000, 0x4, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:343 +0xbace
cmd/compile/internal/gc.typecheck(0xc000408000, 0x4, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.typecheck1(0xc00040e200, 0x1, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:2077 +0x4e65
cmd/compile/internal/gc.typecheck(0xc00040e200, 0x1, 0x0)
	/usr/local/go/src/cmd/compile/internal/gc/typecheck.go:299 +0x785
cmd/compile/internal/gc.Main(0x18c5cf8)
	/usr/local/go/src/cmd/compile/internal/gc/main.go:603 +0x2aa5
main.main()
	/usr/local/go/src/cmd/compile/main.go:52 +0xb1

NOTE: If you remove the field I error, the helpful error appears. I tried changing the type of E to interface{} and I did not get the panic, but the helpful error instead. The same goes for changing E to a concrete type such as int. I've only be able to reproduce with the error type.

[bmizerany]

Another reproduction with odd behavior:

package hrm

// If either of the below types commented out are uncommented, there is no
// panic and a helpful error message.
//
// type HelpfulError1 struct {
//      E int
//      T HelpfulError1
// }    
//      
// type HelpfulError2 struct {
//      E interface{}
//      T HelpfulError2
// }    
        
type Panics struct {
        E error
        T Panics
} 

[cuonglm]

This is fixed on tip, maybe by CL https://go-review.googlesource.com/c/go/+/274094

[bmizerany]

@cuonglm I had a hunch this was a known issue. Thank you for confirming!