Prod builds would explicitly disable this (and other dangerous flags) with `-Wall -Werror` and friends. Shipping with this on is like shipping with incomplete patterns, which would be caught by `-Wall`.

Summary: useful for developing, capital offense for production code.

I'm guessing the kind of people who have Haskell to push into production are not the kind of people who would be inclined to do that.

I'm wondering how soon someone will push some code into production without writing unit tests because they can't be bothered to write unit tests.

People will write bad code regardless of language features. Ignore those people and worry about how people writing good code will use your new feature. If it saves them time, makes their life happier, or lets them write safer code, then it's a win. Similarly, if a feature helps bad programmers avoid today's bad-pattern-du-jour but prevents good programmers from writing good code, you should think twice before adding it. In summary: ignore bad programmers, they can't be saved by language features.

It would be cool if this feature implied no optimizations applied or something along those lines, making it intractable to use for production code.