Add missing T11408.hs

1 files changed, 23 insertions, 0 deletions

diff --git a/testsuite/tests/indexed-types/should_compile/T11408.hs b/testsuite/tests/indexed-types/should_compile/T11408.hs
new file mode 100644
index 0000000000..df63c67a7e
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_compile/T11408.hs
@@ -0,0 +1,23 @@
+{-# LANGUAGE TypeFamilies #-}
+module T11408 where
+
+type family UL a
+type family UR a
+type family MT a b
+
+mkMerge :: a -> UL a -> UR a -> Int
+mkMerge = undefined
+
+merger :: a -> b -> MT a b
+merger = undefined
+
+{-
+merge ::
+ forall a b. (UL (MT a b) ~ a, UR (MT a b) ~ b) => a -> b -> Int
+or
+ forall t. (MT (UL t) (UR t) ~ t) => UL t -> UR t -> Int
+
+These types are equivalent, and in fact neither is ambiguous,
+but the solver has to work quite hard to prove that.
+-}
+merge x y = mkMerge (merger x y) x y