1 files changed, 54 insertions, 0 deletions
diff --git a/testsuite/tests/quantified-constraints/T17267.hs b/testsuite/tests/quantified-constraints/T17267.hs
new file mode 100644
index 0000000000..eaad478003
--- /dev/null
+++ b/testsuite/tests/quantified-constraints/T17267.hs
@@ -0,0 +1,54 @@
+{-# LANGUAGE QuantifiedConstraints #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module T17267 where
+
+class a ~ b => Thing a b
+instance a ~ b => Thing a b
+
+unsafeCoerce :: forall a b. a -> b
+unsafeCoerce a = oops a where
+  oops :: (a ~ b => Thing a b) => (Thing a b => r) -> r
+  oops r = r
+
+
+{-
+-- Now rejected
+class C a b where
+  op :: a -> b
+
+uc :: a -> b
+uc = oops where
+  oops :: (C a b => C a b) => a -> b
+  oops x = op x
+-}
+
+{-
+-- Now rejected
+uc :: a -> b
+uc = oops where
+  oops :: (a ~ b => a ~ b) => a -> b
+  oops x = x
+-}
+
+
+{-
+-- Now rejected
+class C a b where
+  op :: a -> b
+
+class C a b => Thing a b
+instance C a b => Thing a b
+
+unsafeCoerce :: forall a b. a -> b
+unsafeCoerce a = oops (op a :: Thing a b => b)
+  where
+    oops :: (C a b => Thing a b) => (Thing a b => x) -> x
+    oops r = r
+-}
+