He's asking for exhaustiveness checking of pattern matching algebraic data types. Many languages have had this for essentially forever. It does sometimes mean

He's asking for exhaustiveness checking of pattern matching algebraic data types. Many languages have had this for essentially forever. It does sometimes mean that when you update some code, you have to update a bunch of other things. But it's rarely spurious. Usually those other things are broken by failing to consider the new possibility. In the long term, it's a maintenance win.