cmd/compile: "invalid recursive type" error is not as helpful as go/types

[mvdan]

$ go version
go version devel +150bd4ffd4 Wed Sep 23 07:51:17 2020 +0000 linux/amd64
$ cat recursive.go 
package p

type T1 struct {
	f2 T2
}

type T2 struct {
	f1 T1
}
$ go build recursive.go 
# command-line-arguments
./recursive.go:7:6: invalid recursive type T2
$ go build -o gotype $(go env GOROOT)/src/go/types/gotype.go
$ ./gotype recursive.go 
recursive.go:3:6: illegal cycle in declaration of T1
recursive.go:3:6: 	T1 refers to
recursive.go:7:6: 	T2 refers to
recursive.go:3:6: 	T1

I had a cyclic type involving four types across three files, so I really did have to reach for gotype to understand what was going on. I think we should improve the compiler's typechecker to provide a similarly detailed error.

cc @griesemer @mdempsky @cuonglm

[mvdan]

Also cc @findleyr given his recent work on go/types

[griesemer]

We are seriously investigating using a revamped/adjusted go/types for the compiler in the future. The result should be consistent and better error messages from both, the compiler and the type checker.

[gopherbot]

Change https://golang.org/cl/256837 mentions this issue: cmd/compile: make invalid recursive type error for struct clearer

[gopherbot]

Change https://golang.org/cl/258177 mentions this issue: cmd/compile: report type loop for invalid recursive types

[mdempsky]

In Go 1.16, cmd/compile will report:

recursive.go:3:6: invalid recursive type T1
	recursive.go:3:6: T1 refers to
	recursive.go:7:6: T2 refers to
	recursive.go:3:6: T1