Consider: http://play.golang.org/p/UuuyA-zII7

This package compiles in go 1.2 and the GetNum go function returns 42 as expected. In go
1.3 the compilation fails with:

In file included from $WORK/cgobreak/_obj/_cgo_export.c:2:
./code.go:40:30: warning: declaration of 'struct __0' will not be visible outside of
this function [-Wvisibility]
./code.go:40:12: error: conflicting types for 'goCallback'
./code.go:6:6: note: previous declaration is here
/var/folders/52/y5kwxpqj1vqd3n6j1ndnx3200000gn/T/go-build778103108/cgobreak/_obj/_cgo_export.c:8:23:
warning: declaration of 'struct __0' will not be visible outside of this function
[-Wvisibility]
/var/folders/52/y5kwxpqj1vqd3n6j1ndnx3200000gn/T/go-build778103108/cgobreak/_obj/_cgo_export.c:8:5:
error: conflicting types for 'goCallback'
./code.go:6:6: note: previous declaration is here

Comparing the results of manually running cgo of both versions shows that the function
generated by the export directive goes from:

int goCallback(Gen* p0)

in 1.2 to:

int goCallback(struct __0* p0)

Essentially the C-type is ignored and replaced with an unknown, anonymous struct pointer
type which causes clang to fail to compile it.

Tested on darwin/amd64 with both 1.2.2 and tip.