Cannot assign strut to interface type of another package when interface type uses C types

[AlexRouSg]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go1.8.1 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/chotepud/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build397619697=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

package a

import "C"

type interface foo {
    bar() C.Uint32
}

package b

import "C"
import "a"

type struct foo {}
var asdf a.foo = &foo{}

func (*foo) bar() C.Uint32 {
}

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

Code compiles

What did you see instead?

cannot use software literal (type *foo) as type a.foo in assignment:
*software does not implement a.foo (wrong type for bar method)
have bar() C.Uint32
want bar() a.C.Uint32

[AlexRouSg]

Am I not supposed to pass C types between packages?

[dominikh]

Cgo translates C types into equivalent unexported Go types. Because the translations are unexported, a Go package should not expose C types in its exported API: a C type used in one Go package is different from the same C type used in another.

(from https://golang.org/cmd/cgo/)

As far as I can tell, this is working as intended.

[AlexRouSg]

Guess I should've read the docs more carefully ....
Was there any post explaining why?

[ianlancetaylor]

Dup of #13467 , which discusses why cgo works the way it does.