DmdAnal: Recognise precise exceptions from case alternatives (#18086)
Consider ```hs m :: IO () m = do putStrLn "foo" error "bar" ``` `m` (from #18086) always throws a (precise or imprecise) exception or diverges. Yet demand analysis infers `<L,A>` as demand signature instead of `<L,A>x` for it. That's because the demand analyser sees `putStrLn` occuring in a case scrutinee and decides that it has to `deferAfterPreciseException`, because `putStrLn` throws a precise exception on some control flow paths. This will mask the `botDiv` `Divergence`of the single case alt containing `error` to `topDiv`. Since `putStrLn` has `topDiv` itself, the final `Divergence` is `topDiv`. This is easily fixed: `deferAfterPreciseException` works by `lub`ing with the demand type of a virtual case branch denoting the precise exceptional control flow. We used `nopDmdType` before, but we can be more precise and use `exnDmdType`, which is `nopDmdType` with `exnDiv`. Now the `Divergence` from the case alt will degrade `botDiv` to `exnDiv` instead of `topDiv`, which combines with the result from the scrutinee to `exnDiv`, and all is well. Fixes #18086.
Showing with 53 additions and 8 deletions