env GO111MODULE=on

go list -e -f '{{.Incomplete}}' runbad1.go
stdout true
! go run runbad1.go
stderr 'use of internal package m/x/internal not allowed'

go list -e -f '{{.Incomplete}}' runbad2.go
stdout true
! go run runbad2.go
stderr 'use of internal package m/x/internal/y not allowed'

go list -e -f '{{.Incomplete}}' runok.go
stdout false
go run runok.go

-- go.mod --
module m

-- x/internal/internal.go --
package internal

-- x/internal/y/y.go --
package y

-- internal/internal.go --
package internal

-- internal/z/z.go --
package z

-- runbad1.go --
package main
import _ "m/x/internal"
func main() {}

-- runbad2.go --
package main
import _ "m/x/internal/y"
func main() {}

-- runok.go --
package main
import _ "m/internal"
import _ "m/internal/z"
func main() {}