The keyword static has several different meanings in C++, and the code you’ve written above uses them in two different ways.
In the context of member functions, static means “this member function does not have a receiver object. It’s basically a normal function that’s nested inside of the scope of the class.”
In the context of function declarations, static means “this function is scoped only to this file and can’t be called from other places.”
When you implemented the function by writing
static void Foobar::do_something() {} // Error!
the compiler interpreted the static here to mean “I’m implementing this member function, and I want to make that function local just to this file.” That’s not allowed in C++ because it causes some confusion: if multiple different files all defined their own implementation of a member function and then declared them static to avoid collisions at linking, calling the same member function from different places would result in different behavior!
Fortunately, as you noted, there’s an easy fix: just delete the static keyword from the definition:
void Foobar::do_something() {} // Should be good to go!
This is perfectly fine because the compiler already knows that do_something is a static member function, since you told it about that earlier on.