cmd/compile, go/types, types2: unclear error message on initialization loop and invalid parameter type

[gazerro]

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

$ go version
go version go1.16.5 darwin/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="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/marco/Library/Caches/go-build"
GOENV="/Users/marco/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/marco/go/pkg/mod"
GOOS="darwin"
GOPATH="/Users/marco/go"
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="go1.16.5"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/z9/xrln_qks56jbzxjbhs04fpm80000gn/T/go-build1476298263=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Execute this program

package main

var a = f(5)

func f(a) int { _ = a; return 5 }

func main() {}

https://play.golang.org/p/5H8A96e3nn5

What did you expect to see?

./prog.go:3:5: initialization loop:
	prog.go:3:5: a refers to
	prog.go:5:6: f refers to
	prog.go:3:5: a
./prog.go:5:8: a is not a type

What did you see instead?

./prog.go:3:9: typechecking loop involving f
	prog.go:3:10 f(5)
	prog.go:3:5 a = f(5)
	prog.go:3:5 a
	prog.go:5:7 <func>
	prog.go:5:6 f
	prog.go:5:6 <node DCLFUNC>
./prog.go:5:8: a is not a type

With tip the error is

./test.go:3:9: typechecking loop involving f
	prog.go:3:10 f(5)
	prog.go:3:5 a = f(5)
	prog.go:3:5 a
	prog.go:5:7 <func>
	prog.go:5:6 f
	prog.go:5:6 f
./test.go:5:8: a is not a type

[mknyszek]

Is this a recent regression or has it been the case for some time now?

[mknyszek]

I think CC @griesemer for type/semantic checking error messages?

[gazerro]

It is not a recent regression. With Go 1.13 the error is

./prog.go:3:9: typechecking loop involving f
	prog.go:3:10 f(5)
	prog.go:3:5 a = f(5)
	prog.go:o:3:5 a
	prog.go:5:7 <T>
	prog.go:5:6 f
	prog.go:5:6 <node DCLFUNC>
./prog.go:5:8: a is not a type

[griesemer]

Using types2 in the compiler (same for go/types):

$ go tool compile -G=3 x.go
x.go:3:5: invalid recursive type a
	x.go:3:5: a refers to
	x.go:5:6: f refers to
	x.go:3:5: a
x.go:3:5: initialization loop for a
	x.go:3:5: a refers to
	x.go:5:6: f refers to
	x.go:3:5: a

Perhaps slightly better but there should be only one error message.

Thanks for reporting.

Marking for 1.18 in case there's an easy fix, but if not, this is certainly not urgent: we get an error message and we don't crash; also this is a somewhat rare error case.

[griesemer]

Not urgent. Moving to 1.19.

[gazerro]

With version 1.23, the behavior is now working as expected. Therefore, I am closing this issue as it has been resolved.