cmd/cgo: undefined reference for C function with -buildmode=c-archive

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

go version go1.10.3 darwin/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="/Users/Andari/Library/Caches/go-build" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="darwin" GOOS="darwin" GOPATH="/Users/Andari/go" GORACE="" GOROOT="/usr/local/Cellar/go/1.10.3/libexec" GOTMPDIR="" GOTOOLDIR="/usr/local/Cellar/go/1.10.3/libexec/pkg/tool/darwin_amd64" GCCGO="gccgo" CC="clang" CXX="clang++" 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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lw/5pzww_8n38g1lv_kqqmqfvqr0000gn/T/go-build228325343=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

I was experimenting with C interfacing:

// int cadd(int a, int b);
import "C"
...
// using the function somewhere

I built it with go build -buildmode=c-archive GoAdd.go

What did you expect to see?

The build to be successful.

What did you see instead?

Undefined symbols for architecture x86_64: "_cadd", referenced from: __cgo_ece3f0761be9_Cfunc_cadd in _x002.o

Why would linking be neccessary, if I am generating an archive which I link together with the cadd implementing object file later?