./go.mod
./c/c.go
./a/ssa/a.go
./b/ssa/b.go

module modpath

go 1.12

package ssa

type T struct {
}

func (t *T) setNum(n int) {
}

var X interface{} = &T{}

func Run() {
	X.(interface {
		setNum(int)
	}).setNum(0)
}

package ssa

type T struct {
}

func (t *T) setNum(n int) {
}

var X interface{} = &T{}

func Run() {
	X.(interface {
		setNum(int)
	}).setNum(0)
}

package main

import (
	a "modpath/a/ssa"
	b "modpath/b/ssa"
)

func main() {
	a.Run()
	b.Run()
}

cd c
go build
./c

panic: interface conversion: *ssa.T is not interface { ssa.setNum(int) }: missing method setNum

goroutine 1 [running]:
modpath/b/ssa.Run(...)
	/usr/local/google/home/khr/gowork/matloob/b/ssa/b.go:12
main.main()
	/usr/local/google/home/khr/gowork/matloob/c/c.go:10 +0x90