TypeFamilies: should_compile/Simple21

* Code of #1776 from aslatter@gmail.com

testsuite/tests/ghc-regress/indexed-types/should_compile/Simple21.hs

+{-# LANGUAGE TypeFamilies #-}
+
+module ShouldCompile where
+
+import Prelude hiding (foldr, foldr1)
+
+import Data.Maybe
+
+type family Elem x
+
+class Foldable a where
+  foldr :: (Elem a -> b -> b) -> b -> a -> b
+
+  foldr1 :: (Elem a -> Elem a -> Elem a) -> a -> Elem a
+  foldr1 f xs = fromMaybe (error "foldr1: empty structure")
+                  (foldr mf Nothing xs)
+     where mf x Nothing  = Just x
+           mf x (Just y) = Just (f x y)

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

@@ -21,6 +21,7 @@ test('Simple17', expect_fail, compile, [''])
 test('Simple18', normal, compile, [''])
 test('Simple19', normal, compile, [''])
 test('Simple20', normal, compile, [''])
+test('Simple21', normal, compile, [''])
 test('Simple22', normal, compile, [''])
 test('Simple23', normal, compile, [''])