There is actually code in symfmt in cmd/compile/internal/gc/fmt.go that is supposed to handle this (look for If the name was used by multiple packages). I don't know why it is not working for your example. What is your test case?

Here's a test case that can probably be reduced further.

==> ./b/b.go <==
package b
import "a2"

type T struct {
    X a.X
}

==> ./a1/a.go <==
package a

type X string

func NewX() X {
    return ""
}

==> ./a2/a.go <==
package a

type X string

func NewX() X {
    return ""
}

==> ./p/tst.go <==
package main
import (
    "a1"
    "b"
)

var _ = b.T{
    X: a.NewX(),
}

func main() {
}

Sample run:

% pwd
/tmp/d/src/p
% go install
# p
./tst.go:8: cannot use a.NewX() (type a.X) as type a.X in field value

It looks like the respective code in the compiler doesn't trigger because numImport[s.Pkg.Name] is always 0 in this case. Possibly an error in the new importer. Investigating.

Yes, this got lost in the move from the old textual importer to the new binary one. This is a 1.7 regression. We should probably fix it, the fix is simple; with it, the error message is:

tst.go:8: cannot use "/Users/gri/tmp/a".NewX() (type "/Users/gri/tmp/a".X) as type "/Users/gri/tmp/a2".X in field value

(I've compiled all packages in the tmp directory.)
CL coming later today.

@rogpeppe Thanks for the test case. This is in fact a regression since 1.6, so setting milestone to 1.7maybe.

1.6.2:

# issue16133
./tst.go:9: cannot use "issue16133/a1".NewX() (type "issue16133/a1".X) as type "issue16133/a2".X in field value

1.7beta2:

# issue16133
./tst.go:9: cannot use a.NewX() (type a.X) as type a.X in field value

CL https://golang.org/cl/24312 mentions this issue.