Test Trac #7809

testsuite/tests/typecheck/should_fail/T7809.hs

+{-# LANGUAGE RankNTypes, TypeFamilies, LiberalTypeSynonyms #-}
+module T7809 where
+
+type PolyId = (forall a. a -> a)
+
+type family F a
+
+foo :: F PolyId
+foo = undefined
\ No newline at end of file

testsuite/tests/typecheck/should_fail/T7809.stderr

+
+T7809.hs:8:8:
+    Illegal polymorphic or qualified type: PolyId
+    Perhaps you intended to use -XImpredicativeTypes
+    In the type signature for ‛foo’: foo :: F PolyId

testsuite/tests/typecheck/should_fail/all.T

@@ -309,3 +309,4 @@ test('T7851', normal, compile_fail, [''])
 test('T7856', normal, compile_fail, [''])
 test('T7869', normal, compile_fail, [''])
 test('T7892', normal, compile_fail, [''])
+test('T7809', normal, compile_fail, [''])