I removed the parameter from the definition and call sites.

I went from

    void foo(A a, B b) {
       // Only use a
    }
    ...
    foo(x, y);

To this

    void foo(A a) {
       // Only use a
    }
    ...
    foo(x);

And it stopped compiling. There were no other functions called `foo` so it is nothing to do with overloading.

I'll give you the answer if you want or we can leave it a mystery so you can experience how confusing that behaviour is...