Pmc: Do inhabitation test for unlifted vars (#20631)

Although I thought we were already set to handle unlifted datatypes correctly,
it appears we weren't. #20631 showed that it's wrong to assume
`vi_bot=IsNotBot` for `VarInfo`s of unlifted types from their inception if we
don't follow up with an inhabitation test to see if there are any habitable
constructors left. We can't trigger the test from `emptyVarInfo`, so now we
instead fail early in `addBotCt` for variables of unlifted types.

Fixed #20631.

2 files changed, 26 insertions, 0 deletions

diff --git a/testsuite/tests/pmcheck/should_compile/T20631.hs b/testsuite/tests/pmcheck/should_compile/T20631.hs
new file mode 100644
index 0000000000..d255c521ba
--- /dev/null
+++ b/testsuite/tests/pmcheck/should_compile/T20631.hs
@@ -0,0 +1,24 @@
+{-# LANGUAGE UnliftedDatatypes  #-}
+{-# OPTIONS_GHC -fwarn-incomplete-patterns #-}
+module Lib where
+import Data.Kind (Type)
+import GHC.Exts (UnliftedType)
+
+type Foo :: UnliftedType -> Type -> Type
+data Foo a b =
+      Bar a -- no need for strictness annotation
+    | Baz b
+
+type MyVoid :: UnliftedType
+data MyVoid
+
+v :: Foo MyVoid Char
+v = Baz 'c'
+
+main :: IO ()
+main = case v of
+    -- The Baz case is impossible
+    -- MyVoid has no values, and it can't
+    -- be undefined because it's unlifted.
+    Baz c -> putChar c
+
diff --git a/testsuite/tests/pmcheck/should_compile/all.T b/testsuite/tests/pmcheck/should_compile/all.T
index c732ef5691..f1cc928151 100644
--- a/testsuite/tests/pmcheck/should_compile/all.T
+++ b/testsuite/tests/pmcheck/should_compile/all.T
@@ -168,6 +168,8 @@ test('T18932', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 test('T19622', normal, compile,
      ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
+test('T20631', normal, compile,
+     ['-fwarn-incomplete-patterns -fwarn-overlapping-patterns'])
 
 # Other tests
 test('pmc001', [], compile,