Add new test for #8020.

testsuite/tests/indexed-types/should_compile/T8020.hs

+{-# LANGUAGE TypeFamilies #-}
+
+module T8020 where
+
+type family F a b where
+  F (Maybe a) [a] = Int
+  F b c           = Bool
+
+data Proxy a = P
+
+type family G
+
+foo :: Proxy d -> F d d -> Bool
+foo _ = not
+
+bar :: Bool -> Bool
+bar = foo (P :: Proxy G)

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

@@ -226,4 +226,5 @@ test('T8011',
      run_command,
      ['$MAKE -s --no-print-directory T8011'])
 
-test('T8018', normal, compile, [''])
\ No newline at end of file
+test('T8018', normal, compile, [''])
+test('T8020', normal, compile, [''])
\ No newline at end of file