PmCheck: Positive info doesn't imply there is an inhabitant (#18960)

Consider `T18960`:

```hs
pattern P :: a -> a
pattern P x = x
{-# COMPLETE P :: () #-}

foo :: ()
foo = case () of
    P _ -> ()
```

We know about the match variable of the case match that it is equal to `()`.
After the match on `P`, we still know it's equal to `()` (positive info), but
also that it can't be `P` (negative info). By the `COMPLETE` pragma, we know
that implies that the refinement type of the match variable is empty after the
`P` case.

But in the PmCheck solver, we assumed that "has positive info" means
"is not empty", thus assuming we could omit a costly inhabitation test. Which
is wrong, as we saw above.

A bit of a complication arises because the "has positive info" spared us
from doing a lot of inhabitation tests in `T17836b`. So we keep that
check, but give it a lower priority than the check for dirty variables
that requires us doing an inhabitation test.

Needless to say: This doesn't impact soundness of the checker at all,
it just implements a better trade-off between efficiency and precision.

Fixes #18960.

Metric Decrease:
    T17836

1 files changed, 13 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/complete_sigs/T18960.hs b/testsuite/tests/pmcheck/complete_sigs/T18960.hs
new file mode 100644
index 0000000000..ed441647d7
--- /dev/null
+++ b/testsuite/tests/pmcheck/complete_sigs/T18960.hs
@@ -0,0 +1,13 @@
+{-# OPTIONS_GHC -Wincomplete-patterns -fforce-recomp #-}
+{-# LANGUAGE PatternSynonyms #-}
+
+module T18960 where
+
+pattern P :: a -> a
+pattern P x = x
+{-# COMPLETE P :: () #-}
+
+foo :: ()
+foo = case () of
+    P _ -> ()
+