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

$ go version
go version go1.12 linux/s390x

Does this issue reproduce with the latest release?

yes

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

$ go env
GOARCH="s390x"

GOBIN=""

GOCACHE="/root/.cache/go-build"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="s390x"

GOHOSTOS="linux"

GOOS="linux"

GOPATH="/root/go"

GOPROXY=""

GORACE=""

GOROOT="/usr/local/go"

GOTMPDIR=""

GOTOOLDIR="/usr/local/go/pkg/tool/linux_s390x"

GCCGO="gccgo"

CC="s390x-linux-gnu-gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD=""

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS=""

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -march=z196 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build249750574=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have the following structure:

/root/go/src/
|-- foo
    |-- main.go
    |-- types/
        |-- types.go
|-- foo-plugin
    |-- plugin.go

I have an interface in types.go

types/types.go

package types

type Custom interface {
        Test() string
}

I use this in the Plugin

plugin.go

package main

import (
        "foo/types"
)

type CustomStruct struct {
}

var CustomVar types.Custom = &CustomStruct{}

func (c *CustomStruct) Test() string {
        return "success"
}

And i load and use the Plugin in main.go

main.go

package main

import (
        "fmt"
        "plugin"

        "foo/types"
)

func main() {
        p, err := plugin.Open("/root/go/src/foo-plugin/foo-plugin.so")
        if err != nil {
                fmt.Println(err)
        }
        symbol, err := p.Lookup("CustomVar")
        customVar := symbol.(*types.Custom)
        custom := *customVar
        result := custom.Test()
        fmt.Println(result)
}

When i build the Plugin and then run go run main.go it works.
However, when i copy foo into foo-plugin/vendor/foo i get an error.

What did you expect to see?

$ go run main.go
success

What did you see instead?

$ go run main.go
panic: interface conversion: plugin.Symbol is *types.Custom, not *types.Custom (types from different scopes)

goroutine 1 [running]:
main.main()
        /root/go/src/foo/main.go:16 +0x27e
exit status 2