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

go version go1.10 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/USER/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/USER/.gvm/pkgsets/go1.10/global"
GORACE=""
GOROOT="/home/USER/.gvm/gos/go1.10"
GOTMPDIR=""
GOTOOLDIR="/home/USER/.gvm/gos/go1.10/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
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 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build674906758=/tmp/go-build -gno-record-gcc-switches

What did you do?

By using a sample application which uses firebase SDK (https://firebase.google.com/docs/admin/setup)

package main

import (
	"context"
	"log"

	firebase "firebase.google.com/go"
)

func main() {
	if _, err := firebase.NewApp(context.Background(), nil); err != nil {
		log.Fatalf("error initializing app: %v", err)
	}
}

it builds when using dependecies fetched with go get ... but when put in vendor/ directory it breaks

go build
# github.com/pmalek/firebase_dep/vendor/google.golang.org/api/transport/grpc
vendor/google.golang.org/api/transport/grpc/go18.go:25:45: undefined: ocgrpc.ClientHandler

Sample application with vendored out dependencies (using https://github.com/golang/dep) can be found at https://github.com/pmalek/firebase_no_dep

For reasons unknown to me even with build tag

// +build go1.8

the file vendor/google.golang.org/api/transport/grpc/go18.go builds with go1.10 (tested also on go1.9 and the results are the same)

What did you expect to see?

App builds with dependencies fetched with go get and with those put in vendor/ dir.

What did you see instead?

go build
# github.com/pmalek/firebase_dep/vendor/google.golang.org/api/transport/grpc
vendor/google.golang.org/api/transport/grpc/go18.go:25:45: undefined: ocgrpc.ClientHandler