diff --git a/include/boost/unordered/detail/foa_mixer.hpp b/include/boost/unordered/detail/foa_mixer.hpp
new file mode 100644
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+++ b/include/boost/unordered/detail/foa_mixer.hpp
@@ -0,0 +1,175 @@
+/* Hash mixer for boost::unordered::unordered_flat_[map|set].
+ *
+ * Copyright 2022 Joaquin M Lopez Munoz.
+ * Copyright 2022 Peter Dimov.
+ * Distributed under the Boost Software License, Version 1.0.
+ * (See accompanying file LICENSE_1_0.txt or copy at
+ * http://www.boost.org/LICENSE_1_0.txt)
+ *
+ * See https://www.boost.org/libs/unordered for library home page.
+ */
+
+#ifndef BOOST_UNORDERED_DETAIL_FOA_MIXER_HPP
+#define BOOST_UNORDERED_DETAIL_FOA_MIXER_HPP
+
+#include <boost/config.hpp>
+#include <boost/container_hash/hash_fwd.hpp>
+#include <boost/core/empty_value.hpp>
+#include <boost/cstdint.hpp>
+#include <climits>
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+namespace boost{
+namespace unordered{
+namespace detail{
+namespace foa{
+
+/* mixer<Hash> is functionally equivalent to Hash except if Hash is
+ * boost::hash<T> for any of the Ts where boost::hash implements a
+ * trivial hashing function not fit for open-addressing hash container:
+ * in these cases, the result of boost::hash is post-mixed using
+ * 
+ *   - 64 bits: xmx (TODO: Peter Dimov to explain)
+ *   - 32 bits: xmx33 (TODO: Peter Dimov to explain)
+ */
+
+#if defined(BOOST_GCC)
+/* GCC's -Wshadow triggers at scenarios like this: 
+ *
+ *   struct foo{};
+ *   template<typename Base>
+ *   struct derived:Base
+ *   {
+ *     void f(){int foo;}
+ *   };
+ * 
+ *   derived<foo>x;
+ *   x.f(); // declaration of "foo" in derived::f shadows base type "foo"
+ *
+ * This makes shadowing warnings unavoidable in general when a class template
+ * derives from a user-provided class, as is the case with mixer_impl.
+ */
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wshadow"
+#endif
+
+template<typename Hash,typename MixPolicy>
+class mixer_impl:empty_value<Hash>
+{
+public:
+  using base=empty_value<Hash>;
+
+#if BOOST_CXX_VERSION<201703L
+  using argument_type=typename Hash::argument_type;
+  using result_type=std::size_t;
+#endif
+
+  mixer_impl()=default;
+  template<typename... Args>
+  mixer_impl(Args&&... args):base{empty_init,std::forward<Args>(args)...}{}
+
+  Hash&       get_base()noexcept{return base::get();}
+  const Hash& get_base()const noexcept{return base::get();}
+
+  template<
+    typename Key,
+    std::enable_if<
+      std::is_same<
+        std::size_t,
+        decltype(std::declval<const Hash>()(std::declval<const Key&>()))
+      >::value
+    >::type* =nullptr
+  >
+  std::size_t operator()(const Key& x)const
+    noexcept(noexcept(std::declval<const Hash>()(x)))
+  {
+    return MixPolicy::mix(get_base()(x));
+  }
+
+  friend void swap(mixer_impl& x,mixer_impl& y)
+  {
+    using std::swap;
+    swap(x.get_base(),y.get_base());
+  }
+};
+
+#if defined(BOOST_GCC)
+#pragma GCC diagnostic pop /* ignored "-Wshadow" */
+#endif
+
+struct no_mix
+{
+  static inline std::size_t mix(std::size_t x)noexcept{return x;}
+};
+
+#if defined(SIZE_MAX)
+#if ((((SIZE_MAX >> 16) >> 16) >> 16) >> 15) != 0
+#define BOOST_UNORDERED_64B_ARCHITECTURE /* >64 bits assumed as 64 bits */
+#endif
+#elif defined(UINTPTR_MAX) /* used as proxy for std::size_t */
+#if ((((UINTPTR_MAX >> 16) >> 16) >> 16) >> 15) != 0
+#define BOOST_UNORDERED_64B_ARCHITECTURE
+#endif
+#endif
+
+struct xmx_mix
+{
+  static inline std::size_t mix(std::size_t x)noexcept
+  {
+#if defined(BOOST_UNORDERED_64B_ARCHITECTURE)
+
+    boost::uint64_t z=x;
+
+    z^=z>> 23;
+    z*=0xff51afd7ed558ccdull;
+    z^=z>>23;
+
+    return (std::size_t)z;
+
+#else /* 32 bits assumed */
+
+    x^=x>>18;
+    x*=0x56b5aaadu;
+    x^=x>>16;
+
+    return x;
+
+#endif
+  }
+};
+
+#ifdef BOOST_UNORDERED_64B_ARCHITECTURE
+#undef BOOST_UNORDERED_64B_ARCHITECTURE
+#endif
+
+template<typename Hash> struct is_boost_hash:std::false_type{};
+template<typename Key> struct is_boost_hash<boost::hash<Key>>:std::true_type{};
+
+template<typename Hash> struct boost_hash_key_impl{using type=void;};
+template<typename Key> struct boost_hash_key_impl<boost::hash<Key>>
+{
+  using type=Key;
+};
+template<typename Hash> using boost_hash_key=
+  typename boost_hash_key_impl<Hash>::type;
+
+template<typename Hash>
+using mixer=typename std::conditional<
+  is_boost_hash<Hash>::value&&(
+    std::is_integral<boost_hash_key<Hash>>::value||
+    std::is_enum<boost_hash_key<Hash>>::value||
+    std::is_floating_point<boost_hash_key<Hash>>::value|| // TODO: not sure about this one
+    std::is_pointer<boost_hash_key<Hash>>::value),
+  mixer_impl<Hash,xmx_mix>,
+  mixer_impl<Hash,no_mix>
+>::type;
+
+} /* namespace foa */
+} /* namespace detail */
+} /* namespace unordered */
+} /* namespace boost */
+
+#endif