Case split uncovered patterns in warnings or not?

[Sebastian Graf]

Hi Devs,

In https://gitlab.haskell.org/ghc/ghc/-/issues/20642 we saw that GHC >=
8.10 outputs pattern match warnings a little differently than it used to.
Example from there:

{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}foo :: [a] -> [a]foo [] =
[]foo xs = ys  where  (_, ys@(_:_)) = splitAt 0 xsmain :: IO ()main =
putStrLn $ foo $ "Hello, coverage checker!"

Instead of saying

ListPair.hs:7:3: warning: [-Wincomplete-uni-patterns]    Pattern
match(es) are non-exhaustive    In a pattern binding: Patterns not
matched: (_, [])

We now say

ListPair.hs:7:3: warning: [-Wincomplete-uni-patterns]    Pattern
match(es) are non-exhaustive    In a pattern binding:        Patterns
of type ‘([a], [a])’ not matched:            ([], [])
((_:_), [])

E.g., newer versions do (one) case split on pattern variables that haven't
even been scrutinised by the pattern match. That amounts to quantitatively
more pattern suggestions and for each variable a list of constructors that
could be matched on.
The motivation for the change is outlined in
https://gitlab.haskell.org/ghc/ghc/-/issues/20642#note_390110, but I could
easily be swayed not to do the case split. Which arguably is less
surprising, as Andreas Abel points out.

Considering the other examples from my post, which would you prefer?

Cheers,
Sebastian
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