Pmc: Don't case split on wildcard matches (#20642)

Since 8.10, when formatting a pattern match warning, we'd case split on a
wildcard match such as
```hs
foo :: [a] -> [a]
foo [] = []
foo xs = ys
  where
  (_, ys@(_:_)) = splitAt 0 xs
-- Pattern match(es) are non-exhaustive
-- In a pattern binding:
--     Patterns not matched:
--         ([], [])
--         ((_:_), [])
```
But that's quite verbose and distracts from which part of the pattern was
actually the inexhaustive one. We'd prefer a wildcard for the first pair
component here, like it used to be in GHC 8.8.

On the other hand, case splitting is pretty handy for `-XEmptyCase` to know the
different constructors we could've matched on:
```hs
f :: Bool -> ()
f x = case x of {}
-- Pattern match(es) are non-exhaustive
-- In a pattern binding:
--     Patterns not matched:
--         False
--         True
```
The solution is to communicate that we want a top-level case split to
`generateInhabitingPatterns` for `-XEmptyCase`, which is exactly what
this patch arranges. Details in `Note [Case split inhabiting patterns]`.

Fixes #20642.

2 files changed, 4 insertions, 4 deletions

diff --git a/testsuite/tests/th/T19709c.stderr b/testsuite/tests/th/T19709c.stderr
index 3bedc08dc9..37451d2023 100644
--- a/testsuite/tests/th/T19709c.stderr
+++ b/testsuite/tests/th/T19709c.stderr
@@ -4,7 +4,7 @@ T19709c.hs:9:7: error: [-Wincomplete-patterns (in -Wextra), -Werror=incomplete-p
     In a case alternative:
         Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'h'}
-            (p:_:_) where p is not one of {'h'}
+            (p:_) where p is not one of {'h'}
             ['h']
+            ('h':p:_) where p is not one of {'e'}
             ...
diff --git a/testsuite/tests/th/T19709d.stderr b/testsuite/tests/th/T19709d.stderr
index 3ee2ac8424..cacf6acb48 100644
--- a/testsuite/tests/th/T19709d.stderr
+++ b/testsuite/tests/th/T19709d.stderr
@@ -4,9 +4,9 @@ T19709d.hs:6:2: warning: [-Wincomplete-patterns (in -Wextra)]
     In a case alternative:
         Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'h'}
-            (p:_:_) where p is not one of {'h'}
+            (p:_) where p is not one of {'h'}
             ['h']
+            ('h':p:_) where p is not one of {'e'}
             ...
 
 T19709d.hs:1:1: error: