C archive from CGo with function dependency on a C function

I am trying to build a C-archive from a Go-file with an external dependency on a C-function. I try to use the following command

GOOS=darwin GOARCH=amd64 CGO_ENABLED=1 go build -buildmode=c-archive -o demo.a demo.go

The problem I encounter is that the go build command demands that foo() is defined. Am I fundamentally missing something regarding how Go works? How do I build a static library with dependencies?

The error from go build

Undefined symbols for architecture x86_64:
"_foo", referenced from:
__cgo_849fe01848a8_Cfunc_foo in demo.cgo2.o
(maybe you meant: __cgo_849fe01848a8_Cfunc_foo)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1

My demo.go looks like the following

package demo

extern int foo();
import "C"

import (

//export afunc
func afunc() {
    fmt.Println("Hello from afunc")

I am aware that I can get around this using function pointers, but I would still like to know why a step in the go build process demands that the function in question is defined when building a static library.

Any ideas?

This should do the trick:

extern int foo();
#pragma weak foo
import "C"

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