The warnings were about NoMonoLocalBinds making type inference fragile: tests/V1/tests.hs:1876:17: warning: [-Wsimplifiable-class-constraints] • The constraint ‘SOP.All SOP.SListI (SOP.GCode a)’ matches an instance declaration instance forall k (f :: k -> Constraint) (xs :: [k]). (Generics.SOP.Constraint.AllF f xs, SOP.SListI xs) => SOP.All f xs -- Defined in ‘Generics.SOP.Constraint’ This makes type inference for inner bindings fragile; either use MonoLocalBinds, or simplify it using the instance • In the type signature: sopArbitrary :: forall a. (Generic a, SOP.GTo a, SOP.All SOP.SListI (SOP.GCode a), SOP.All2 Arbitrary (SOP.GCode a)) => Gen a