1 files changed, 39 insertions, 0 deletions
diff --git a/testsuite/tests/typecheck/should_compile/tc216.hs b/testsuite/tests/typecheck/should_compile/tc216.hs
new file mode 100644
index 0000000000..4a23f3df7f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/tc216.hs
@@ -0,0 +1,39 @@
+{-# LANGUAGE UndecidableInstances, FlexibleInstances,
+             MultiParamTypeClasses, FunctionalDependencies #-}
+
+-- Test for trac #816
+-- GHC's typechecker loops when trying to type this, resulting in a
+-- context stack overflow.
+
+{- Maybe this should typecheck:
+
+   Given: Foo x y, Bar y z
+   Wanted: Foo x beta, Bar beta z
+
+If we happened to process (Foo x beta) first we 
+might generate the extra equality beta~y, and we are good
+
+If we process (Bar beta z) first, we end up in an infinite
+loop, using the (Bar x z) instance repeatedly.
+
+If instead we'd had
+   class (F x ~ y) => Foo x y where
+     type F x
+     foo :: x -> y
+
+Then after canonicalising we get
+     Given: Foo x y, Bar y z, F x ~ y
+     Wanted: Foo x beta, Bar beta z
+-}
+
+module ShouldCompile where
+
+class Foo x y | x -> y where
+ foo :: x -> y
+
+class Bar x z where
+ bar :: x -> z -> Int
+
+instance (Foo x y, Bar y z) => Bar x z where
+ bar x z = bar (foo x) z
+