env GO111MODULE=off

! go build canonical/d
stderr '^canonical[/\\]b[/\\]b.go:3:8: non-canonical import path "canonical/a/": should be "canonical/a"$'

-- canonical/a/a.go --
package a

import _ "c"
-- canonical/b/b.go --
package b

import _ "canonical/a/"
-- canonical/a/vendor/c/c.go --
package c
-- canonical/d/d.go --
package d

import _ "canonical/b"