Add test case for #7938.

testsuite/tests/indexed-types/should_fail/T7938.hs

+{-# LANGUAGE TypeFamilies, PolyKinds, MultiParamTypeClasses,
+             FlexibleInstances, DataKinds #-}
+
+module T7938 where
+
+data KProxy (a :: *) = KP
+
+class Foo (a :: k1) (b :: k2) where
+  type Bar a
+
+instance Foo (a :: k1) (b :: k2) where
+  type Bar a = (KP :: KProxy k2)
\ No newline at end of file

testsuite/tests/indexed-types/should_fail/T7938.stderr

+
+T7938.hs:12:16:
+    Expected kind ‛*’, but ‛KP’ has kind ‛KProxy k2’
+    In the type ‛(KP :: KProxy k2)’
+    In the type instance declaration for ‛Bar’
+    In the instance declaration for ‛Foo (a :: k1) (b :: k2)’

testsuite/tests/indexed-types/should_fail/all.T

@@ -100,3 +100,4 @@ test('T7786', normal, compile_fail, [''])
 test('NoGood', normal, compile_fail, [''])
 test('T7967', normal, compile_fail, [''])
 
+test('T7938', normal, compile_fail, [''])
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