go/types, types2: stack overflow results after naming type parameter after interface name.

[bmizerany]

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

$ go version
go version go1.18beta1 darwin/arm64

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/bmizerany/Library/Caches/go-build"
GOENV="/Users/bmizerany/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/bmizerany/go/pkg/mod"
GONOPROXY="tier.run"
GONOSUMDB="tier.run"
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_arm64"
GOVCS=""
GOVERSION="go1.18beta1"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/bmizerany/go/src/exhausted/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/rf/6fdgwbmj2rxf1jw13q4fv7h80000gn/T/go-build4012310581=/tmp/go-build -gno-record-gcc-switches -fno-common"
GOROOT/bin/go version: go version go1.18beta1 darwin/arm64
GOROOT/bin/go tool compile -V: compile version go1.18beta1
uname -v: Darwin Kernel Version 21.1.0: Wed Oct 13 17:33:24 PDT 2021; root:xnu-8019.41.5~1/RELEASE_ARM64_T8101
ProductName:	macOS
ProductVersion:	12.0.1
BuildVersion:	21A559
lldb --version: lldb-1300.0.32.4
Swift version 5.5.1-dev

What did you do?

I wrote this package that names a type parameter the same as its interface and ran `go build .'

exhausted % /bin/cat exhausted.go 
package issue

type I interface {
	int | string
}

func foo[I I](v Thing) {}

What did you expect to see?

It build or provide a helpful error about what I did wrong, especially something around Thing because it is undefined, but it seems Go flipped a switch before it noticed.

What did you see instead?

# blake.io/exausted/cmd/exhausted
runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0x14020460390 stack=[0x14020460000, 0x14040460000]
fatal error: stack overflow

runtime stack:
runtime.throw({0x100dcd86d?, 0x10145a500?})
	/usr/local/go/src/runtime/panic.go:992 +0x50
runtime.newstack()
	/usr/local/go/src/runtime/stack.go:1101 +0x46c
runtime.morestack()
	/usr/local/go/src/runtime/asm_arm64.s:310 +0x70

goroutine 1 [running]:
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:99 +0x23c fp=0x14020460390 sp=0x14020460390 pc=0x100d0977c
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x140204603b0 sp=0x14020460390 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x140204603d0 sp=0x140204603b0 pc=0x100d088c8
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:104 +0x38 fp=0x14020460430 sp=0x140204603d0 pc=0x100d09578
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x14020460450 sp=0x14020460430 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x14020460470 sp=0x14020460450 pc=0x100d088c8
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:104 +0x38 fp=0x140204604d0 sp=0x14020460470 pc=0x100d09578
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x140204604f0 sp=0x140204604d0 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x14020460510 sp=0x140204604f0 pc=0x100d088c8
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:104 +0x38 fp=0x14020460570 sp=0x14020460510 pc=0x100d09578
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x14020460590 sp=0x14020460570 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x140204605b0 sp=0x14020460590 pc=0x100d088c8
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:104 +0x38 fp=0x14020460610 sp=0x140204605b0 pc=0x100d09578
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x14020460630 sp=0x14020460610 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x14020460650 sp=0x14020460630 pc=0x100d088c8
cmd/compile/internal/types2.(*TypeParam).iface(0x140003d7350)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:104 +0x38 fp=0x140204606b0 sp=0x14020460650 pc=0x100d09578
cmd/compile/internal/types2.(*TypeParam).Underlying(0x0?)
	/usr/local/go/src/cmd/compile/internal/types2/typeparam.go:88 +0x20 fp=0x140204606d0 sp=0x140204606b0 pc=0x100d094b0
cmd/compile/internal/types2.under({0x101006548?, 0x140003d7350?})
	/usr/local/go/src/cmd/compile/internal/types2/type.go:27 +0x68 fp=0x140204606f0 sp=0x140204606d0 pc=0x100d088c8

...additional frames elided...

This may possibly be related to or a duplicate of: #48619.

[ianlancetaylor]

CC @griesemer @findleyr

[griesemer]

This is fixed at tip. For

package issue

type I interface {
	int | string
}

func foo[I I](v I) {}

the compiler correctly reports:

cannot use a type parameter as constraint

Closing.