Add a test for Trac #13951

It no longer gives a warning.

2 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T13951.hs b/testsuite/tests/typecheck/should_compile/T13951.hs
new file mode 100644
index 0000000000..8cbeb8c11a
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T13951.hs
@@ -0,0 +1,19 @@
+{-# LANGUAGE PolyKinds, GADTs, Rank2Types, ScopedTypeVariables, Trustworthy #-}
+module Control.Monad.Skeleton.Internal where
+
+data Cat k a b where
+  Empty :: Cat k a a
+  Leaf :: k a b -> Cat k a b
+  Tree :: Cat k a b -> Cat k b c -> Cat k a c
+
+viewL :: forall k a b r. Cat k a b
+  -> ((a ~ b) => r)
+  -> (forall x. k a x -> Cat k x b -> r)
+  -> r
+viewL Empty e _ = e
+viewL (Leaf k) _ r = k `r` Empty
+viewL (Tree a b) e r = go a b where
+  go :: Cat k a x -> Cat k x b -> r
+  go Empty t = viewL t e r
+  go (Leaf k) t = r k t
+  go (Tree c d) t = go c (Tree d t)
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index d312f5074d..b94f021058 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -669,3 +669,4 @@ test('T16188', normal, compile, [''])
 test('T16204a', normal, compile, [''])
 test('T16204b', normal, compile, [''])
 test('T16225', normal, compile, [''])
+test('T13951', normal, compile, [''])