Linking error when assigning a struct static field outside of it

Question

This simple code

struct Foo {
    static double bar;
};



int main() {
    Foo::bar = 15.0;
}

fails with the following error:

Undefined symbols for architecture x86_64:
  "Foo::bar", referenced from:
      _main in experiment-0579ea.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I don't understand why this doesn't work