PmCheck: Better long-distance info for where bindings (#18533)

Where bindings can see evidence from the pattern match of the `GRHSs`
they belong to, but not from anything in any of the guards (which belong
to one of possibly many RHSs).

Before this patch, we did *not* consider said evidence, causing #18533,
where the lack of considering type information from a case pattern match
leads to failure to resolve the vanilla COMPLETE set of a data type.

Making available that information required a medium amount of
refactoring so that `checkMatches` can return a
`[(Deltas, NonEmpty Deltas)]`; one `(Deltas, NonEmpty Deltas)` for each
`GRHSs` of the match group. The first component of the pair is the
covered set of the pattern, the second component is one covered set per
RHS.

Fixes #18533.
Regression test case: T18533

2 files changed, 26 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T18533.hs b/testsuite/tests/pmcheck/should_compile/T18533.hs
new file mode 100644
index 0000000000..3b4fdf8b89
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T18533.hs
@@ -0,0 +1,24 @@
+{-# OPTIONS_GHC -Wincomplete-patterns -fforce-recomp #-}
+{-# LANGUAGE GADTs, DataKinds, TypeFamilies, BangPatterns #-}
+
+module T18533 where
+
+data SBool (b :: Bool) where
+  STrue  :: SBool 'True
+  SFalse :: SBool 'False
+
+type family Fam (b :: Bool)
+type instance Fam 'True = T
+
+data T = T Bool
+
+f :: Fam b -> SBool b -> Bool
+f !t s = case s of
+    STrue -> a where a = case t of T a -> a
+    _     -> False
+
+
+g :: Bool -> Bool
+g x = case x of
+  True -> a where a = case x of True -> False
+  False -> True
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index 57527977ff..054ad82873 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -122,6 +122,8 @@ test('T18049', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T18478', collect_compiler_stats('bytes allocated',10), compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T18533', normal, compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 
 # Other tests
 test('pmc001', [], compile,