Adding test for #5120

testsuite/tests/ghc-regress/typecheck/should_compile/T5120.hs

+{-# LANGUAGE ImplicitParams #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE NoMonomorphismRestriction #-}
+
+module Test where
+
+class C t where
+    type TF t
+    ttt :: TF t -> t
+
+b :: (C t, ?x :: TF t) => t
+b = ttt ?x 

testsuite/tests/ghc-regress/typecheck/should_compile/all.T

@@ -342,3 +342,4 @@ test('T4912', extra_clean(['T4912a.hi', 'T4912a.o']),
 
 test('T4952', normal, compile, [''])
 test('T4969', normal, compile, [''])
+test('T5120', normal, compile, [''])
\ No newline at end of file