IE For "List list" to work, there must be a thing named List available as a typedef. For list = 0 to work, tlist must be a pointer or an int. For list->data to work, list must be a pointer to a struct, and that struct must have a member named data. for list->data = val to work, the member named data must be compatible with a double.

It simply generates all of the constraints like this on the program, and then solves the constraint set, which produces a valid program (hopefully. it is possible it may not have enough data in some cases, given how C is parsed)