cmd/compile: No error when using interface with typeset with variable

[icholy]

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

$ go version
go version go1.18 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="/home/icholy/.cache/go-build"
GOENV="/home/icholy/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/icholy/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/icholy/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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 -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build722358823=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

func main() {
	var _ interface{ string | any } = "test"
}

https://go.dev/play/p/L9p1kcNJs9O

What did you expect to see?

./prog.go:4:8: interface contains type constraints

Go build failed.

What did you see instead?

Compiled successfully.

[mdempsky]

What's the expected behavior here? I didn't think interface{ string | any } was valid as a normal variable type, but semantically I guess it denotes the same interface as interface{}?

/cc @griesemer @ianlancetaylor @findleyr

[mdempsky]

Spec says "Interfaces whose type sets can be defined entirely by a list of methods are called basic interfaces."

So that suggests to me that it's intended that interface{ string | any } is a basic interface type, and thus should be valid as a variable type. And the compiler is correct to accept it (i.e., this issue can be closed without further action).

Related #52124.

[griesemer]

@mdempsky Correct. This interface is the same as interface{} and thus permitted in this context.

[gopherbot]

Change https://go.dev/cl/398094 mentions this issue: test: extend issue52124.go to also test #52139