-- | Manufacture a coercion from this air. Needless to say, this is not usually safe,
-- but it is used when we know we are dealing with bottom, which is one case in which
-- it is safe. This is also used implement the @unsafeCoerce#@ primitive.
+-- Optimise by pushing down through type constructors
mkUnsafeCoercion :: Type -> Type -> Coercion
-mkUnsafeCoercion ty1 ty2 = mkCoercion unsafeCoercionTyCon [ty1, ty2]
+mkUnsafeCoercion (TyConApp tc1 tys1) (TyConApp tc2 tys2)
+ | tc1 == tc2
+ = TyConApp tc1 (zipWith mkUnsafeCoercion tys1 tys2)
+mkUnsafeCoercion (FunTy a1 r1) (FunTy a2 r2)
+ = FunTy (mkUnsafeCoercion a1 a2) (mkUnsafeCoercion r1 r2)
+
+mkUnsafeCoercion ty1 ty2
+ | ty1 `coreEqType` ty2 = ty1
+ | otherwise = mkCoercion unsafeCoercionTyCon [ty1, ty2]
-- See note [Newtype coercions] in TyCon