diff --git a/include/boost/mp11/detail/mp_defer.hpp b/include/boost/mp11/detail/mp_defer.hpp
new file mode 100644
index 0000000..9aaca99
--- /dev/null
+++ b/include/boost/mp11/detail/mp_defer.hpp
@@ -0,0 +1,119 @@
+#ifndef BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED
+#define BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED
+
+// Copyright 2015-2020, 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/integral.hpp>
+#include <boost/mp11/detail/config.hpp>
+
+namespace boost
+{
+namespace mp11
+{
+
+// mp_if, mp_if_c
+namespace detail
+{
+
+template<bool C, class T, class... E> struct mp_if_c_impl
+{
+};
+
+template<class T, class... E> struct mp_if_c_impl<true, T, E...>
+{
+    using type = T;
+};
+
+template<class T, class E> struct mp_if_c_impl<false, T, E>
+{
+    using type = E;
+};
+
+} // namespace detail
+
+template<bool C, class T, class... E> using mp_if_c = typename detail::mp_if_c_impl<C, T, E...>::type;
+template<class C, class T, class... E> using mp_if = typename detail::mp_if_c_impl<static_cast<bool>(C::value), T, E...>::type;
+
+// mp_valid
+
+#if BOOST_MP11_WORKAROUND( BOOST_MP11_INTEL, != 0 ) // tested at 1800
+
+// contributed by Roland Schulz in https://github.com/boostorg/mp11/issues/17
+
+namespace detail
+{
+
+template<class...> using void_t = void;
+
+template<class, template<class...> class F, class... T>
+struct mp_valid_impl: mp_false {};
+
+template<template<class...> class F, class... T>
+struct mp_valid_impl<void_t<F<T...>>, F, T...>: mp_true {};
+
+} // namespace detail
+
+template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<void, F, T...>;
+
+#else
+
+// implementation by Bruno Dutra (by the name is_evaluable)
+namespace detail
+{
+
+template<template<class...> class F, class... T> struct mp_valid_impl
+{
+    template<template<class...> class G, class = G<T...>> static mp_true check(int);
+    template<template<class...> class> static mp_false check(...);
+
+    using type = decltype(check<F>(0));
+};
+
+} // namespace detail
+
+template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<F, T...>::type;
+
+#endif
+
+template<class Q, class... T> using mp_valid_q = mp_valid<Q::template fn, T...>;
+
+// mp_defer
+namespace detail
+{
+
+template<template<class...> class F, class... T> struct mp_defer_impl
+{
+    using type = F<T...>;
+};
+
+struct mp_no_type
+{
+};
+
+#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )
+
+template<template<class...> class F, class... T> struct mp_defer_cuda_workaround
+{
+    using type = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
+};
+
+#endif
+
+} // namespace detail
+
+#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )
+
+template<template<class...> class F, class... T> using mp_defer = typename detail::mp_defer_cuda_workaround< F, T...>::type;
+
+#else
+
+template<template<class...> class F, class... T> using mp_defer = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
+
+#endif
+
+} // namespace mp11
+} // namespace boost
+
+#endif // #ifndef BOOST_MP11_DETAIL_MP_DEFER_HPP_INCLUDED
diff --git a/include/boost/mp11/detail/mp_fold.hpp b/include/boost/mp11/detail/mp_fold.hpp
index 0745e87..266d9c1 100644
--- a/include/boost/mp11/detail/mp_fold.hpp
+++ b/include/boost/mp11/detail/mp_fold.hpp
@@ -9,6 +9,7 @@
 //  http://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/mp11/detail/config.hpp>
+#include <boost/mp11/detail/mp_defer.hpp>
 
 namespace boost
 {
@@ -41,12 +42,113 @@ template<template<class...> class L, class V, template<class...> class F> struct
 
 #endif
 
-template<template<class...> class L, class T1, class... T, class V, template<class...> class F> struct mp_fold_impl<L<T1, T...>, V, F>
+//
+
+template<class V, template<class...> class F> struct mp_fold_Q1
 {
-    using type = typename mp_fold_impl<L<T...>, F<V, T1>, F>::type;
+    template<class T1>
+        using fn = F<V, T1>;
 };
 
-template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class T9, class T10, class... T, class V, template<class...> class F> struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T...>, V, F>
+template<class V, template<class...> class F> struct mp_fold_Q2
+{
+    template<class T1, class T2>
+        using fn = F<F<V, T1>, T2>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q3
+{
+    template<class T1, class T2, class T3>
+        using fn = F<F<F<V, T1>, T2>, T3>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q4
+{
+    template<class T1, class T2, class T3, class T4>
+        using fn = F<F<F<F<V, T1>, T2>, T3>, T4>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q5
+{
+    template<class T1, class T2, class T3, class T4, class T5>
+        using fn = F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q6
+{
+    template<class T1, class T2, class T3, class T4, class T5, class T6>
+        using fn = F<F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>, T6>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q7
+{
+    template<class T1, class T2, class T3, class T4, class T5, class T6, class T7>
+        using fn = F<F<F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>, T6>, T7>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q8
+{
+    template<class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8>
+        using fn = F<F<F<F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>, T6>, T7>, T8>;
+};
+
+template<class V, template<class...> class F> struct mp_fold_Q9
+{
+    template<class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class T9>
+        using fn = F<F<F<F<F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>, T6>, T7>, T8>, T9>;
+};
+
+//
+
+template<template<class...> class L, class T1, class V, template<class...> class F>
+struct mp_fold_impl<L<T1>, V, F>: mp_defer<mp_fold_Q1<V, F>::template fn, T1>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2>, V, F>: mp_defer<mp_fold_Q2<V, F>::template fn, T1, T2>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3>, V, F>: mp_defer<mp_fold_Q3<V, F>::template fn, T1, T2, T3>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4>, V, F>: mp_defer<mp_fold_Q4<V, F>::template fn, T1, T2, T3, T4>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5>, V, F>: mp_defer<mp_fold_Q5<V, F>::template fn, T1, T2, T3, T4, T5>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6>, V, F>: mp_defer<mp_fold_Q6<V, F>::template fn, T1, T2, T3, T4, T5, T6>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class T7, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6, T7>, V, F>: mp_defer<mp_fold_Q7<V, F>::template fn, T1, T2, T3, T4, T5, T6, T7>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6, T7, T8>, V, F>: mp_defer<mp_fold_Q8<V, F>::template fn, T1, T2, T3, T4, T5, T6, T7, T8>
+{
+};
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class T9, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6, T7, T8, T9>, V, F>: mp_defer<mp_fold_Q9<V, F>::template fn, T1, T2, T3, T4, T5, T6, T7, T8, T9>
+{
+};
+
+//
+
+template<template<class...> class L, class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class T9, class T10, class... T, class V, template<class...> class F>
+struct mp_fold_impl<L<T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T...>, V, F>
 {
     using type = typename mp_fold_impl<L<T...>, F<F<F<F<F<F<F<F<F<F<V, T1>, T2>, T3>, T4>, T5>, T6>, T7>, T8>, T9>, T10>, F>::type;
 };
diff --git a/include/boost/mp11/utility.hpp b/include/boost/mp11/utility.hpp
index fbab4a3..4010aee 100644
--- a/include/boost/mp11/utility.hpp
+++ b/include/boost/mp11/utility.hpp
@@ -13,6 +13,7 @@
 #include <boost/mp11/detail/mp_fold.hpp>
 #include <boost/mp11/detail/mp_front.hpp>
 #include <boost/mp11/detail/mp_rename.hpp>
+#include <boost/mp11/detail/mp_defer.hpp>
 #include <boost/mp11/detail/config.hpp>
 
 namespace boost
@@ -33,104 +34,9 @@ template<class T> using mp_identity_t = typename mp_identity<T>::type;
 template<class... T> struct mp_inherit: T... {};
 
 // mp_if, mp_if_c
-namespace detail
-{
-
-template<bool C, class T, class... E> struct mp_if_c_impl
-{
-};
-
-template<class T, class... E> struct mp_if_c_impl<true, T, E...>
-{
-    using type = T;
-};
-
-template<class T, class E> struct mp_if_c_impl<false, T, E>
-{
-    using type = E;
-};
-
-} // namespace detail
-
-template<bool C, class T, class... E> using mp_if_c = typename detail::mp_if_c_impl<C, T, E...>::type;
-template<class C, class T, class... E> using mp_if = typename detail::mp_if_c_impl<static_cast<bool>(C::value), T, E...>::type;
-
 // mp_valid
-
-#if BOOST_MP11_WORKAROUND( BOOST_MP11_INTEL, != 0 ) // tested at 1800
-
-// contributed by Roland Schulz in https://github.com/boostorg/mp11/issues/17
-
-namespace detail
-{
-
-template<class...> using void_t = void;
-
-template<class, template<class...> class F, class... T>
-struct mp_valid_impl: mp_false {};
-
-template<template<class...> class F, class... T>
-struct mp_valid_impl<void_t<F<T...>>, F, T...>: mp_true {};
-
-} // namespace detail
-
-template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<void, F, T...>;
-
-#else
-
-// implementation by Bruno Dutra (by the name is_evaluable)
-namespace detail
-{
-
-template<template<class...> class F, class... T> struct mp_valid_impl
-{
-    template<template<class...> class G, class = G<T...>> static mp_true check(int);
-    template<template<class...> class> static mp_false check(...);
-
-    using type = decltype(check<F>(0));
-};
-
-} // namespace detail
-
-template<template<class...> class F, class... T> using mp_valid = typename detail::mp_valid_impl<F, T...>::type;
-
-#endif
-
-template<class Q, class... T> using mp_valid_q = mp_valid<Q::template fn, T...>;
-
 // mp_defer
-namespace detail
-{
-
-template<template<class...> class F, class... T> struct mp_defer_impl
-{
-    using type = F<T...>;
-};
-
-struct mp_no_type
-{
-};
-
-#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )
-
-template<template<class...> class F, class... T> struct mp_defer_cuda_workaround
-{
-    using type = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
-};
-
-#endif
-
-} // namespace detail
-
-#if BOOST_MP11_WORKAROUND( BOOST_MP11_CUDA, >= 9000000 && BOOST_MP11_CUDA < 10000000 )
-
-template<template<class...> class F, class... T> using mp_defer = typename detail::mp_defer_cuda_workaround< F, T...>::type;
-
-#else
-
-template<template<class...> class F, class... T> using mp_defer = mp_if<mp_valid<F, T...>, detail::mp_defer_impl<F, T...>, detail::mp_no_type>;
-
-#endif
+//   moved to detail/mp_defer.hpp
 
 // mp_eval_if, mp_eval_if_c
 namespace detail
diff --git a/test/Jamfile b/test/Jamfile
index 0e25055..28ce8e9 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -88,6 +88,7 @@ run mp_find_if_q.cpp ;
 run mp_reverse.cpp ;
 run mp_fold.cpp ;
 run mp_fold_q.cpp ;
+run mp_fold_q_sf.cpp ;
 run mp_reverse_fold.cpp ;
 run mp_reverse_fold_q.cpp ;
 run mp_unique.cpp ;
diff --git a/test/mp_fold_q_sf.cpp b/test/mp_fold_q_sf.cpp
new file mode 100644
index 0000000..cd116ab
--- /dev/null
+++ b/test/mp_fold_q_sf.cpp
@@ -0,0 +1,50 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/mp11/algorithm.hpp>
+#include <boost/mp11/list.hpp>
+#include <boost/mp11/integral.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <type_traits>
+#include <tuple>
+
+using boost::mp11::mp_size_t;
+
+struct Q
+{
+    template<class N, class T> using fn = mp_size_t<N::value + sizeof(T)>;
+};
+
+int main()
+{
+    using boost::mp11::mp_valid;
+    using boost::mp11::mp_fold_q;
+    using boost::mp11::mp_list;
+
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_fold_q, mp_list<>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_fold_q, mp_list<int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_fold_q, mp_list<int, int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_fold_q, mp_list<int, int, int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid<mp_fold_q, mp_list<int, int, int, int>, mp_size_t<0>, Q>));
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<void>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, void>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, int, void>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, int, int, void>, mp_size_t<0>, Q>));
+
+#if !BOOST_MP11_WORKAROUND( BOOST_MP11_MSVC, < 1910 )
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<void, int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<void, int, int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<void, int, int, int>, mp_size_t<0>, Q>));
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, void, int>, mp_size_t<0>, Q>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, void, int, int>, mp_size_t<0>, Q>));
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid<mp_fold_q, mp_list<int, int, void, int>, mp_size_t<0>, Q>));
+
+#endif
+
+    return boost::report_errors();
+}