Add test case for #8776

testsuite/tests/ghci/scripts/T8776.hs

+{-# LANGUAGE PatternSynonyms, GADTs #-}
+data A x y = (Num x, Eq y) => B
+
+data R = R{ rX :: Int }
+
+pattern P = B

testsuite/tests/ghci/scripts/T8776.script

+:load T8776.hs
+:i P

testsuite/tests/ghci/scripts/T8776.stdout

+pattern (Num t, Eq t1) => P :: (A t t1) 	-- Defined at T8776.hs:6:9

testsuite/tests/ghci/scripts/all.T

@@ -166,3 +166,5 @@ test('T8579', normal, ghci_script, ['T8579.script'])
 test('T8649', normal, ghci_script, ['T8649.script'])
 test('T8674', normal, ghci_script, ['T8674.script'])
 test('T8696', normal, ghci_script, ['T8696.script'])
+test('T8776', normal, ghci_script, ['T8776.script'])
+