Disallow class instances for synonyms

See Trac #13267 and Note [Instances and constraint synonyms]
in TcValidity.

We can't easily do a perfect job, because the rename is really trying
to do its lookup too early.  But this is at least an improvement.

1 files changed, 10 insertions, 0 deletions

diff --git a/testsuite/tests/polykinds/T13267.hs b/testsuite/tests/polykinds/T13267.hs
new file mode 100644
index 0000000000..cfc7efbffc
--- /dev/null
+++ b/testsuite/tests/polykinds/T13267.hs
@@ -0,0 +1,10 @@
+{-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-}
+module T13267 where
+
+type C1 a = (Show (a -> Bool))
+
+instance C1 Int where
+
+type C2 a = (Show Bool, Show Int)
+
+instance C2 Int where