At the limit, the answer is “between zero and completely.” Zero because you may only have access to the prototype and not the body, say if the body is in anothe

At the limit, the answer is “between zero and completely.” Zero because you may only have access to the prototype and not the body, say if the body is in another translation unit, or completely if a full solution could be found, which is certainly possible for trivial cases. The reason to not do this isn’t due to impossibility, but for other factors: it’s computationally expensive, I’d you think compile times are already bad, get ready for them to get way worse. Also, changing the body can break code in competently different parts of your program, as changing the body changes the signature and can now invalidate callers.