Zonk the returned kind in tcFamTyPatswip/T19250

The motivation is given in Note [tcFamTyPats: zonking the result kind].

Fixes #19250  --  the fix is easy.

2 files changed, 13 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T19250.hs b/testsuite/tests/polykinds/T19250.hs
new file mode 100644
index 0000000000..6781e4bd39
--- /dev/null
+++ b/testsuite/tests/polykinds/T19250.hs
@@ -0,0 +1,12 @@
+{-# LANGUAGE TypeOperators, TypeFamilies, ConstraintKinds, PolyKinds, DataKinds, EmptyDataDecls #-}
+
+module T19250 where
+
+import Data.Kind
+
+type Exp a = a -> Type
+
+type family Eval (e :: Exp a) :: a
+
+data Collapse :: [Constraint] -> Exp Constraint
+type instance Eval (Collapse '[]) = ()
diff --git a/testsuite/tests/polykinds/all.T b/testsuite/tests/polykinds/all.T
index c82f275f65..1be9bb11b5 100644
--- a/testsuite/tests/polykinds/all.T
+++ b/testsuite/tests/polykinds/all.T
@@ -234,3 +234,4 @@ test('T18855', normal, compile, [''])
 test('T19092', normal, compile, [''])
 test('T19093', normal, compile, [''])
 test('T19094', normal, compile, [''])
+test('T19250', normal, compile, [''])