The code generator is unable to generate valid C code if a method with varargs also declares to throw an error, i.e.

void func(int val, ...) throws GLib.Error {}

generates:

void func(int val, ..., GError *error) {}

This occurs whether the function is declared within a method, static, extern'ed. Of particular note is that if defined in a vapi file, it is correctly able to call the function (i.e. call with error before the varargs).