Smallest example of a weird build error "nested/sub.Foo: undefined: nested/sub.bar"

foo.foo.go

// file: nested/foo/foo.go

package main
import "nested/sub"

func main() {
	nestedsub.Foo()
}

nested.go

// file: nested/nested.go

package nested

// theoretical stuff 

sub.nested.c

// file: nested/sub/nested.c

#include <runtime.h>

void
nestedsub·bar() {
    runtime·prints("hello");
}

sub.nested.go

// file: nested/sub/nested.go

package nestedsub

func bar()
func Foo() {
	bar()
}