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.

28 files changed, 134 insertions, 112 deletions

diff --git a/testsuite/tests/pmcheck/complete_sigs/T18960b.stderr b/testsuite/tests/pmcheck/complete_sigs/T18960b.stderr
index fd27f0853e..6148f75a07 100644
--- a/testsuite/tests/pmcheck/complete_sigs/T18960b.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/T18960b.stderr
@@ -5,8 +5,8 @@ T18960b.hs:11:7: warning: [-Wincomplete-patterns (in -Wextra)]
         Patterns of type ‘((), String)’ not matched:
             (_, _)
             P ((), [])
-            P ((), [p]) where p is not one of {'h'}
-            P ((), (p:_:_)) where p is not one of {'h'}
+            P ((), (p:_)) where p is not one of {'h'}
+            P ((), ['h'])
             ...
 
 T18960b.hs:18:7: warning: [-Wincomplete-patterns (in -Wextra)]
@@ -15,6 +15,6 @@ T18960b.hs:18:7: warning: [-Wincomplete-patterns (in -Wextra)]
         Patterns of type ‘((), String)’ not matched:
             (_, _)
             P ((), [])
-            P ((), [p]) where p is not one of {'h'}
-            P ((), (p:_:_)) where p is not one of {'h'}
+            P ((), (p:_)) where p is not one of {'h'}
+            P ((), ['h'])
             ...
diff --git a/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr b/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
index b993966789..2d58080949 100644
--- a/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
+++ b/testsuite/tests/pmcheck/complete_sigs/completesig06.stderr
@@ -1,29 +1,29 @@
 
 completesig06.hs:13:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In an equation for ‘m1’: Patterns of type  ‘T’ not matched: B
+    In an equation for ‘m1’: Patterns of type ‘T’ not matched: B
 
 completesig06.hs:16:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In an equation for ‘m2’: Patterns of type  ‘T’ not matched: A
+    In an equation for ‘m2’: Patterns of type ‘T’ not matched: A
 
 completesig06.hs:20:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘m3’:
-        Patterns of type  ‘T’ not matched:
+        Patterns of type ‘T’ not matched:
             A
             B
 
 completesig06.hs:23:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘m4’:
-        Patterns of type  ‘T’, ‘S’ not matched:
+        Patterns of type ‘T’, ‘S’ not matched:
             A D
             B D
 
 completesig06.hs:29:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘m5’:
-        Patterns of type  ‘T’, ‘S’ not matched:
-            A D
-            B D
+        Patterns of type ‘T’, ‘S’ not matched:
+            A _
+            B _
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase001.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase001.stderr
index e3794c0405..cd00f26f8b 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase001.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase001.stderr
@@ -1,15 +1,15 @@
 
 EmptyCase001.hs:9:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘Int’ not matched: _
+    In a case alternative: Patterns of type ‘Int’ not matched: _
 
 EmptyCase001.hs:14:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘String’ not matched:
+        Patterns of type ‘String’ not matched:
             []
             (_:_)
 
 EmptyCase001.hs:18:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘Char’ not matched: _
+    In a case alternative: Patterns of type ‘Char’ not matched: _
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase002.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase002.stderr
index af9411c6a3..691c62b79d 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase002.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase002.stderr
@@ -1,25 +1,25 @@
 
 EmptyCase002.hs:16:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘T’ not matched: MkT _
+    In a case alternative: Patterns of type ‘T’ not matched: MkT _
 
 EmptyCase002.hs:43:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T1 B’ not matched:
+        Patterns of type ‘T1 B’ not matched:
             MkT1 B1
             MkT1 B2
 
 EmptyCase002.hs:47:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T1 (E Int)’ not matched:
+        Patterns of type ‘T1 (E Int)’ not matched:
             MkT1 False
             MkT1 True
 
 EmptyCase002.hs:51:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T1
-                             (T2 (T1 (D (E Int) (E (E Int)))))’ not matched:
+        Patterns of type ‘T1
+                            (T2 (T1 (D (E Int) (E (E Int)))))’ not matched:
             MkT1 (MkT2 (MkT1 D2))
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase004.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase004.stderr
index c80a391d98..7dc717c934 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase004.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase004.stderr
@@ -1,37 +1,37 @@
 
 EmptyCase004.hs:15:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘A Bool’ not matched: A2
+    In a case alternative: Patterns of type ‘A Bool’ not matched: A2
 
 EmptyCase004.hs:19:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘A a’ not matched:
+        Patterns of type ‘A a’ not matched:
             A1
             A2
 
 EmptyCase004.hs:31:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘B a a’ not matched: B1 _
+    In a case alternative: Patterns of type ‘B a a’ not matched: B1 _
 
 EmptyCase004.hs:35:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘B a b’ not matched:
+        Patterns of type ‘B a b’ not matched:
             B1 _
             B2
 
 EmptyCase004.hs:47:6: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘A a’ not matched:
+        Patterns of type ‘A a’ not matched:
             A1
             A2
 
 EmptyCase004.hs:50:9: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘B a b’ not matched: B2
+    In a case alternative: Patterns of type ‘B a b’ not matched: B2
 
 EmptyCase004.hs:51:9: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘B a b’ not matched: B1 _
+    In a case alternative: Patterns of type ‘B a b’ not matched: B1 _
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
index 97514f67be..da479135f6 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase005.stderr
@@ -2,34 +2,34 @@
 EmptyCase005.hs:24:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Void3’ not matched: Void3 _
+        Patterns of type ‘Void3’ not matched: Void3 _
 
 EmptyCase005.hs:67:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T ()’ not matched:
+        Patterns of type ‘T ()’ not matched:
             T1
             T2
 
 EmptyCase005.hs:73:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T Bool’ not matched:
+        Patterns of type ‘T Bool’ not matched:
             MkTBool False
             MkTBool True
 
 EmptyCase005.hs:79:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘T Int’ not matched: MkTInt _
+        Patterns of type ‘T Int’ not matched: MkTInt _
 
 EmptyCase005.hs:91:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘G Int’ not matched:
+        Patterns of type ‘G Int’ not matched:
             MkV False
             MkV True
 
 EmptyCase005.hs:101:8: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘H Int’ not matched: _
+    In a case alternative: Patterns of type ‘H Int’ not matched: _
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase006.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase006.stderr
index a34d0f5e5e..9062f1c40a 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase006.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase006.stderr
@@ -2,12 +2,12 @@
 EmptyCase006.hs:18:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo1 Int’ not matched: Foo1 MkGA1
+        Patterns of type ‘Foo1 Int’ not matched: Foo1 MkGA1
 
 EmptyCase006.hs:26:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo1 a’ not matched:
+        Patterns of type ‘Foo1 a’ not matched:
             Foo1 MkGA1
             Foo1 (MkGA2 _)
             Foo1 MkGA3
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase007.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase007.stderr
index 6b60fd76ad..7adef0854a 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase007.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase007.stderr
@@ -2,30 +2,30 @@
 EmptyCase007.hs:21:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo2 a’ not matched: Foo2 _
+        Patterns of type ‘Foo2 a’ not matched: Foo2 _
 
 EmptyCase007.hs:25:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo2 (a, a)’ not matched: Foo2 _
+        Patterns of type ‘Foo2 (a, a)’ not matched: Foo2 _
 
 EmptyCase007.hs:33:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo2 Int’ not matched: Foo2 (_, _)
+        Patterns of type ‘Foo2 Int’ not matched: Foo2 (_, _)
 
 EmptyCase007.hs:37:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo2 Char’ not matched: Foo2 _
+        Patterns of type ‘Foo2 Char’ not matched: Foo2 _
 
 EmptyCase007.hs:44:17: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘FA Char’ not matched: _
+    In a case alternative: Patterns of type ‘FA Char’ not matched: _
 
 EmptyCase007.hs:48:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo2 [Int]’ not matched:
+        Patterns of type ‘Foo2 [Int]’ not matched:
             Foo2 []
             Foo2 (_:_)
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase008.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase008.stderr
index 5563a599b6..c826a05569 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase008.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase008.stderr
@@ -2,21 +2,21 @@
 EmptyCase008.hs:17:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo3 Int’ not matched:
+        Patterns of type ‘Foo3 Int’ not matched:
             Foo3 (MkDA1 _)
             Foo3 MkDA2
 
 EmptyCase008.hs:21:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo3 a’ not matched: Foo3 _
+        Patterns of type ‘Foo3 a’ not matched: Foo3 _
 
 EmptyCase008.hs:40:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo4 Int ()’ not matched: Foo4 MkDB1
+        Patterns of type ‘Foo4 Int ()’ not matched: Foo4 MkDB1
 
 EmptyCase008.hs:48:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Foo4 a b’ not matched: Foo4 _
+        Patterns of type ‘Foo4 a b’ not matched: Foo4 _
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase009.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase009.stderr
index cd10631c6c..ca6ca03e9f 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase009.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase009.stderr
@@ -1,14 +1,14 @@
 
 EmptyCase009.hs:21:9: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘Bar f’ not matched: Bar _
+    In a case alternative: Patterns of type ‘Bar f’ not matched: Bar _
 
 EmptyCase009.hs:33:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Bar (DB ())’ not matched: Bar MkDB2_u
+        Patterns of type ‘Bar (DB ())’ not matched: Bar MkDB2_u
 
 EmptyCase009.hs:42:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Bar GB’ not matched: Bar MkGB3
+        Patterns of type ‘Bar GB’ not matched: Bar MkGB3
diff --git a/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr b/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr
index 9b4c65e4ac..8202c65a22 100644
--- a/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr
+++ b/testsuite/tests/pmcheck/should_compile/EmptyCase010.stderr
@@ -2,31 +2,31 @@
 EmptyCase010.hs:24:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz GC a’ not matched:
+        Patterns of type ‘Baz GC a’ not matched:
             Baz MkGC1
             Baz (MkGC2 _)
 
 EmptyCase010.hs:28:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz GC 'T1’ not matched: Baz MkGC1
+        Patterns of type ‘Baz GC 'T1’ not matched: Baz MkGC1
 
 EmptyCase010.hs:37:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz GD Maybe’ not matched:
+        Patterns of type ‘Baz GD Maybe’ not matched:
             Baz MkGD1
             Baz MkGD3
 
 EmptyCase010.hs:41:9: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz GD (Either Int)’ not matched: Baz MkGD3
+        Patterns of type ‘Baz GD (Either Int)’ not matched: Baz MkGD3
 
 EmptyCase010.hs:45:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz GD f’ not matched:
+        Patterns of type ‘Baz GD f’ not matched:
             Baz MkGD1
             Baz MkGD2
             Baz MkGD3
@@ -34,14 +34,14 @@ EmptyCase010.hs:45:7: warning: [-Wincomplete-patterns (in -Wextra)]
 EmptyCase010.hs:57:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz (DC ()) a’ not matched: Baz _
+        Patterns of type ‘Baz (DC ()) a’ not matched: Baz _
 
 EmptyCase010.hs:69:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz (DC Bool) [Int]’ not matched: Baz MkDC2
+        Patterns of type ‘Baz (DC Bool) [Int]’ not matched: Baz MkDC2
 
 EmptyCase010.hs:73:9: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Baz f a’ not matched: Baz _
+        Patterns of type ‘Baz f a’ not matched: Baz _
diff --git a/testsuite/tests/pmcheck/should_compile/T10746.stderr b/testsuite/tests/pmcheck/should_compile/T10746.stderr
index fab96e5fde..a2229d3ec6 100644
--- a/testsuite/tests/pmcheck/should_compile/T10746.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T10746.stderr
@@ -2,6 +2,6 @@
 T10746.hs:9:10: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Bool’ not matched:
+        Patterns of type ‘Bool’ not matched:
             False
             True
diff --git a/testsuite/tests/pmcheck/should_compile/T11336b.stderr b/testsuite/tests/pmcheck/should_compile/T11336b.stderr
index 4a6547d509..85dad6a81c 100644
--- a/testsuite/tests/pmcheck/should_compile/T11336b.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T11336b.stderr
@@ -1,5 +1,4 @@
 
 T11336b.hs:25:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In an equation for ‘fun’:
-        Patterns of type  ‘Proxy a’ not matched: Proxy
+    In an equation for ‘fun’: Patterns of type ‘Proxy a’ not matched: _
diff --git a/testsuite/tests/pmcheck/should_compile/T11822.stderr b/testsuite/tests/pmcheck/should_compile/T11822.stderr
index 212d300537..7f0aae9aec 100644
--- a/testsuite/tests/pmcheck/should_compile/T11822.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T11822.stderr
@@ -2,16 +2,15 @@
 T11822.hs:33:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘mkTreeNode’:
-        Patterns of type  ‘prefix’, ‘Seq SiblingDependencies’,
-                          ‘Set prefix’, ‘Depth’ not matched:
-            _ (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT)
-            (Data.Set.Internal.Bin _ _ _ _) (Depth _)
-            _ (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT)
-            Data.Set.Internal.Tip (Depth _)
-            _ (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Single _))
-            (Data.Set.Internal.Bin _ _ _ _) (Depth _)
-            _ (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Single _))
-            Data.Set.Internal.Tip (Depth _)
+        Patterns of type ‘prefix’, ‘Seq SiblingDependencies’, ‘Set prefix’,
+                         ‘Depth’ not matched:
+            _ (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT) _ _
+            _ (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Single _)) _
+            _
+            _
+            (Data.Sequence.Internal.Seq (Data.Sequence.Internal.Deep _ _ _ _))
+            _ _
+            _ (Data.Sequence.Internal.Seq Data.Sequence.Internal.EmptyT) _ _
             ...
 
 T11822.hs:33:1: warning:
@@ -21,4 +20,3 @@ T11822.hs:33:1: warning:
       • Patterns reported as unmatched might actually be matched
     Suggested fix:
       Increase the limit or resolve the warnings to suppress this message.
-
diff --git a/testsuite/tests/pmcheck/should_compile/T15305.stderr b/testsuite/tests/pmcheck/should_compile/T15305.stderr
index de682464b7..61f47322a0 100644
--- a/testsuite/tests/pmcheck/should_compile/T15305.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T15305.stderr
@@ -2,4 +2,4 @@
 T15305.hs:48:23: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Abyss’ not matched: MkAbyss _
+        Patterns of type ‘Abyss’ not matched: MkAbyss _
diff --git a/testsuite/tests/pmcheck/should_compile/T15450.stderr b/testsuite/tests/pmcheck/should_compile/T15450.stderr
index 2ef488970d..a9268100f1 100644
--- a/testsuite/tests/pmcheck/should_compile/T15450.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T15450.stderr
@@ -2,10 +2,10 @@
 T15450.hs:6:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘Bool’ not matched:
+        Patterns of type ‘Bool’ not matched:
             False
             True
 
 T15450.hs:9:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In a case alternative: Patterns of type  ‘Bool’ not matched: False
+    In a case alternative: Patterns of type ‘Bool’ not matched: False
diff --git a/testsuite/tests/pmcheck/should_compile/T17977.stderr b/testsuite/tests/pmcheck/should_compile/T17977.stderr
index f9cb656f42..9817638af6 100644
--- a/testsuite/tests/pmcheck/should_compile/T17977.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T17977.stderr
@@ -2,10 +2,7 @@
 T17977.hs:31:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’:
-        Patterns of type  ‘SNat m’, ‘SNat n’, ‘SNat o’,
-                          ‘R m n o’ not matched:
-            SZ SZ SZ _
-            SZ SZ (SS _) _
-            SZ (SS _) SZ _
-            SZ (SS _) (SS _) _
-            ...
+        Patterns of type ‘SNat m’, ‘SNat n’, ‘SNat o’,
+                         ‘R m n o’ not matched:
+            SZ _ _ _
+            (SS _) _ SZ _
diff --git a/testsuite/tests/pmcheck/should_compile/T18610.stderr b/testsuite/tests/pmcheck/should_compile/T18610.stderr
index 7f6a2dfe67..cc2ddf2996 100644
--- a/testsuite/tests/pmcheck/should_compile/T18610.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T18610.stderr
@@ -6,7 +6,7 @@ T18610.hs:15:3: warning: [-Woverlapping-patterns (in -Wdefault)]
 T18610.hs:24:7: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type ‘(Bool, Bool)’ not matched: (_, _)
+        Patterns of type ‘(Bool, Bool)’ not matched: _
 
 T18610.hs:53:3: warning: [-Winaccessible-code (in -Wdefault)]
     • Couldn't match type ‘Bool’ with ‘Int’
diff --git a/testsuite/tests/pmcheck/should_compile/T18932.stderr b/testsuite/tests/pmcheck/should_compile/T18932.stderr
index 7f6d813ada..1da153f795 100644
--- a/testsuite/tests/pmcheck/should_compile/T18932.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T18932.stderr
@@ -5,15 +5,12 @@ T18932.hs:10:1: warning: [-Woverlapping-patterns (in -Wdefault)]
 
 T18932.hs:12:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In an equation for ‘f'’:
-        Patterns of type  ‘T a’ not matched: MkT2 _
+    In an equation for ‘f'’: Patterns of type ‘T a’ not matched: MkT2 _
 
 T18932.hs:14:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’:
-        Patterns of type  ‘T a’, ‘T a’, ‘T a’ not matched:
-            (MkT2 _) (MkT1 _) (MkT1 _)
-            (MkT2 _) (MkT1 _) (MkT2 _)
-            (MkT2 _) (MkT2 _) (MkT1 _)
-            (MkT2 _) (MkT2 _) (MkT2 _)
-            ...
+        Patterns of type ‘T a’, ‘T a’, ‘T a’ not matched:
+            (MkT2 _) _ _
+            (MkT1 _) (MkT2 _) _
+            (MkT1 _) (MkT1 _) (MkT2 _)
diff --git a/testsuite/tests/pmcheck/should_compile/T20642.hs b/testsuite/tests/pmcheck/should_compile/T20642.hs
new file mode 100644
index 0000000000..f7b1867b67
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T20642.hs
@@ -0,0 +1,18 @@
+{-# LANGUAGE EmptyCase #-}
+{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}
+
+module T20642 where
+
+foo :: [a] -> [a]
+foo [] = []
+foo xs = ys
+  where
+  (_, ys@(_:_)) = splitAt 0 xs
+
+-- Should case split on the Maybe
+f :: Maybe a -> ()
+f x = case x of {}
+
+-- Should *not* case split on the Bool
+g :: (Bool, [a]) -> ()
+g (_, []) = ()
diff --git a/testsuite/tests/pmcheck/should_compile/T20642.stderr b/testsuite/tests/pmcheck/should_compile/T20642.stderr
new file mode 100644
index 0000000000..3843b53045
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T20642.stderr
@@ -0,0 +1,17 @@
+
+T20642.hs:10:3: warning: [-Wincomplete-uni-patterns (in -Wall)]
+    Pattern match(es) are non-exhaustive
+    In a pattern binding:
+        Patterns of type ‘([a], [a])’ not matched: (_, [])
+
+T20642.hs:14:7: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In a case alternative:
+        Patterns of type ‘Maybe a’ not matched:
+            Nothing
+            Just _
+
+T20642.hs:18:1: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In an equation for ‘g’:
+        Patterns of type ‘(Bool, [a])’ not matched: (_, (_:_))
diff --git a/testsuite/tests/pmcheck/should_compile/T2204.stderr b/testsuite/tests/pmcheck/should_compile/T2204.stderr
index 82988e669c..5bd3dba322 100644
--- a/testsuite/tests/pmcheck/should_compile/T2204.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T2204.stderr
@@ -2,14 +2,14 @@
 T2204.hs:6:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’:
-        Patterns of type  ‘String’ not matched:
+        Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'0'}
-            (p:_:_) where p is not one of {'0'}
+            (p:_) where p is not one of {'0'}
             ['0']
+            ('0':p:_) where p is not one of {'1'}
             ...
 
 T2204.hs:9:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’:
-        Patterns of type  ‘Int’ not matched: p where p is not one of {0}
+        Patterns of type ‘Int’ not matched: p where p is not one of {0}
diff --git a/testsuite/tests/pmcheck/should_compile/T9951b.stderr b/testsuite/tests/pmcheck/should_compile/T9951b.stderr
index 65198db65f..c446661366 100644
--- a/testsuite/tests/pmcheck/should_compile/T9951b.stderr
+++ b/testsuite/tests/pmcheck/should_compile/T9951b.stderr
@@ -2,9 +2,9 @@
 T9951b.hs:7:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’:
-        Patterns of type  ‘String’ not matched:
+        Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'a'}
-            (p:_:_) where p is not one of {'a'}
+            (p:_) where p is not one of {'a'}
             ['a']
+            ('a':p:_) where p is not one of {'b'}
             ...
diff --git a/testsuite/tests/pmcheck/should_compile/TooManyDeltas.stderr b/testsuite/tests/pmcheck/should_compile/TooManyDeltas.stderr
index 9297e1b669..bd9ca3f0f0 100644
--- a/testsuite/tests/pmcheck/should_compile/TooManyDeltas.stderr
+++ b/testsuite/tests/pmcheck/should_compile/TooManyDeltas.stderr
@@ -1,12 +1,7 @@
 
 TooManyDeltas.hs:14:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
-    In an equation for ‘f’:
-        Patterns of type  ‘T’, ‘T’ not matched:
-            A A
-            A B
-            B A
-            B B
+    In an equation for ‘f’: Patterns of type ‘T’, ‘T’ not matched: _ _
 
 TooManyDeltas.hs:14:1: warning:
     Pattern match checker ran into -fmax-pmcheck-models=0 limit, so
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index f1cc928151..8df4d33d8a 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -170,6 +170,8 @@ test('T19622', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T20631', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T20642', normal, compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 
 # Other tests
 test('pmc001', [], compile,
diff --git a/testsuite/tests/pmcheck/should_compile/pmc001.stderr b/testsuite/tests/pmcheck/should_compile/pmc001.stderr
index 6f15444777..4e10c5bf0a 100644
--- a/testsuite/tests/pmcheck/should_compile/pmc001.stderr
+++ b/testsuite/tests/pmcheck/should_compile/pmc001.stderr
@@ -2,7 +2,7 @@
 pmc001.hs:14:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’:
-        Patterns of type  ‘T [a]’, ‘T [a]’ not matched:
+        Patterns of type ‘T [a]’, ‘T [a]’ not matched:
             MkT1 MkT3
             (MkT2 _) MkT3
             MkT3 MkT1
@@ -11,7 +11,7 @@ pmc001.hs:14:1: warning: [-Wincomplete-patterns (in -Wextra)]
 pmc001.hs:19:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’:
-        Patterns of type  ‘T [a]’, ‘T [a]’ not matched:
+        Patterns of type ‘T [a]’, ‘T [a]’ not matched:
             MkT1 MkT3
             (MkT2 _) MkT3
             MkT3 MkT1
diff --git a/testsuite/tests/pmcheck/should_compile/pmc007.stderr b/testsuite/tests/pmcheck/should_compile/pmc007.stderr
index 1593f85f7f..a68618d3f2 100644
--- a/testsuite/tests/pmcheck/should_compile/pmc007.stderr
+++ b/testsuite/tests/pmcheck/should_compile/pmc007.stderr
@@ -2,25 +2,25 @@
 pmc007.hs:7:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’:
-        Patterns of type  ‘a’ not matched:
+        Patterns of type ‘a’ not matched:
             p where p is not one of {"ab", "ac"}
 
 pmc007.hs:12:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’:
-        Patterns of type  ‘String’ not matched:
+        Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'a'}
-            (p:_:_) where p is not one of {'a'}
+            (p:_) where p is not one of {'a'}
             ['a']
+            ('a':p:_) where p is not one of {'b', 'c'}
             ...
 
 pmc007.hs:18:11: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In a case alternative:
-        Patterns of type  ‘String’ not matched:
+        Patterns of type ‘String’ not matched:
             []
-            [p] where p is not one of {'a'}
-            (p:_:_) where p is not one of {'a'}
+            (p:_) where p is not one of {'a'}
             ['a']
+            ('a':p:_) where p is not one of {'b', 'c'}
             ...
diff --git a/testsuite/tests/pmcheck/should_compile/pmc009.stderr b/testsuite/tests/pmcheck/should_compile/pmc009.stderr
index 6f1849a25f..21132abc08 100644
--- a/testsuite/tests/pmcheck/should_compile/pmc009.stderr
+++ b/testsuite/tests/pmcheck/should_compile/pmc009.stderr
@@ -2,5 +2,4 @@
 pmc009.hs:6:1: warning: [-Wincomplete-patterns (in -Wextra)]
     Pattern match(es) are non-exhaustive
     In an equation for ‘addPatSynSelector’:
-        Patterns of type  ‘GenLocated l (HsBindLR idL idR)’ not matched:
-            L _ _
+        Patterns of type ‘GenLocated l (HsBindLR idL idR)’ not matched: _