Add a regression test for #11822

The particular test is already fixed, but the issue seems to have
multiple different test cases lumped together.

testsuite/tests/pmcheck/should_compile/T11822.hs

+{-# OPTIONS_GHC -Wincomplete-patterns -fforce-recomp #-}
+{-# LANGUAGE OverloadedLists #-}
+{-# LANGUAGE PatternSynonyms #-}
+{-# LANGUAGE GeneralisedNewtypeDeriving #-}
+
+module T11822 where
+
+import Data.Sequence (Seq, pattern (:<|))
+import Data.Set (Set)
+
+newtype SiblingDependencies = SiblingDependencies Int
+  deriving (Eq, Ord, Enum, Integral, Real, Num)
+
+newtype Depth = Depth Int
+  deriving (Eq, Ord, Enum, Integral, Real, Num)
+
+data TreeNode prefix
+  = OnlyChild prefix
+  | LeafLast prefix
+  | LeafMid prefix
+  | NodeLast prefix
+  | NodeMid prefix
+  | PrefixedLast prefix (Seq SiblingDependencies) (Set prefix) Depth
+  | PrefixedMid prefix (Seq SiblingDependencies) (Set prefix) Depth
+
+mkTreeNode
+  :: Ord prefix
+  => prefix
+  -> Seq SiblingDependencies
+  -> Set prefix
+  -> Depth
+  -> TreeNode prefix
+mkTreeNode t     []      _   _   = OnlyChild t
+mkTreeNode t     [0]    []   _   = LeafLast t
+mkTreeNode t     [_]    []   _   = LeafMid t
+mkTreeNode t     [0]     _   0   = LeafLast t
+mkTreeNode t     [_]     _   0   = LeafMid t
+mkTreeNode t     [0]     _   _   = NodeLast t
+mkTreeNode t     [_]     _   _   = NodeMid t
+mkTreeNode t (0 :<| ns) ds depth = PrefixedLast t ns ds depth
+mkTreeNode t (_ :<| ns) ds depth = PrefixedMid t ns ds depth
+

testsuite/tests/pmcheck/should_compile/T11822.stderr

+
+T11822.hs:33:1: warning: [-Wincomplete-patterns (in -Wextra)]
+    Pattern match(es) are non-exhaustive
+    In an equation for ‘mkTreeNode’:
+        Patterns not matched:
+            _ (Data.Sequence.Internal.Seq _) _ _
+            _ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
+            _ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
+            _ (Data.Sequence.Internal.Seq _) _ p where p is not one of {0}
+            ...

testsuite/tests/pmcheck/should_compile/all.T

@@ -40,16 +40,14 @@ test('T11303', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
 test('T11276', collect_compiler_stats('bytes allocated',10), compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
-
 test('T11303b', collect_compiler_stats('bytes allocated',10), compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
-
 test('T11374', collect_compiler_stats('bytes allocated',10), compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M1G -RTS'])
-
+test('T11822', collect_compiler_stats('bytes allocated',10), compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T11195', collect_compiler_stats('bytes allocated',10), compile,
      ['-package ghc -fwarn-incomplete-patterns -fwarn-overlapping-patterns +RTS -M2G -RTS'])
-
 test('T11984', normal, compile,
     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T14086', normal, compile,