What steps will reproduce the problem?
1. gccgo -c bork.go

with bork.go:

package bork
type Z interface {
        X(y *Y)
}

What is the expected output?

not an internal compiler error

What do you see instead?

bork.go:3:7: error: use of undefined type ‘Y’
go1: internal compiler error: tree check: expected class ‘type’, have
‘exceptional’ (error_mark) in do_get_tree, at go/gofrontend/types.cc:6903
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html>; for instructions.

Which compiler are you using (5g, 6g, 8g, gccgo)?

gccgo

Which operating system are you using?

linux

Which revision are you using?  (hg identify)

URL: svn://gcc.gnu.org/svn/gcc/trunk
Repository Root: svn://gcc.gnu.org/svn/gcc
Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4
Revision: 167582