diff --git a/include/boost/system/detail/error_category.hpp b/include/boost/system/detail/error_category.hpp
index 8b5ba51..b91c193 100644
--- a/include/boost/system/detail/error_category.hpp
+++ b/include/boost/system/detail/error_category.hpp
@@ -172,6 +172,7 @@ namespace detail
 
 static const boost::ulong_long_type generic_category_id = ( boost::ulong_long_type( 0xB2AB117A ) << 32 ) + 0x257EDF0D;
 static const boost::ulong_long_type system_category_id = ( boost::ulong_long_type( 0x8FAFD21E ) << 32 ) + 0x25C5E09B;
+static const boost::ulong_long_type interop_category_id = ( boost::ulong_long_type( 0x943F2817 ) << 32 ) + 0xFD3A8FAF;
 
 BOOST_SYSTEM_CONSTEXPR inline bool failed_impl( int ev, error_category const & cat )
 {
diff --git a/include/boost/system/detail/error_code.hpp b/include/boost/system/detail/error_code.hpp
index 2089ecb..97da236 100644
--- a/include/boost/system/detail/error_code.hpp
+++ b/include/boost/system/detail/error_code.hpp
@@ -10,15 +10,24 @@
 //
 //  See library home page at http://www.boost.org/libs/system
 
+#include <boost/system/is_error_code_enum.hpp>
 #include <boost/system/detail/error_category.hpp>
 #include <boost/system/detail/error_condition.hpp>
 #include <boost/system/detail/system_category.hpp>
+#include <boost/system/detail/system_category_impl.hpp>
+#include <boost/system/detail/interop_category.hpp>
 #include <boost/system/detail/enable_if.hpp>
-#include <boost/system/is_error_code_enum.hpp>
+#include <boost/system/detail/is_same.hpp>
+#include <boost/system/detail/snprintf.hpp>
 #include <boost/system/detail/config.hpp>
 #include <boost/cstdint.hpp>
 #include <boost/config.hpp>
 #include <ostream>
+#include <new>
+
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+# include <system_error>
+#endif
 
 namespace boost
 {
@@ -35,26 +44,70 @@ namespace system
 //  and error_code containing a pointer to an object of a type derived
 //  from error_category.
 
+namespace detail
+{
+
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+typedef std::error_code std_error_code;
+
+#else
+
+class std_error_code
+{
+private:
+
+    struct category_type;
+
+    int val_;
+    category_type const* cat_;
+
+public:
+
+    int value() const { return val_; }
+    category_type const& category() const { return *cat_; }
+};
+
+#endif
+
+} // namespace detail
+
 class error_code
 {
 private:
 
-    int val_;
-    bool failed_;
-    const error_category * cat_;
+    struct data
+    {
+        int val_;
+        const error_category * cat_;
+    };
+
+    union
+    {
+        data d1_;
+        unsigned char d2_[ sizeof(detail::std_error_code) ];
+    };
+
+    // 0: default constructed, d1_ value initialized
+    // 1: holds std::error_code in d2_
+    // 2: holds error code in d1_, failed == false
+    // 3: holds error code in d1_, failed == true
+    unsigned flags_;
 
 public:
 
     // constructors:
 
     BOOST_SYSTEM_CONSTEXPR error_code() BOOST_NOEXCEPT:
-        val_( 0 ), failed_( false ), cat_( &system_category() )
+        d1_(), flags_( 0 )
     {
     }
 
     BOOST_SYSTEM_CONSTEXPR error_code( int val, const error_category & cat ) BOOST_NOEXCEPT:
-        val_( val ), failed_( detail::failed_impl( val, cat ) ), cat_( &cat )
+        d1_(), flags_( 2 + detail::failed_impl( val, cat ) )
     {
+        d1_.val_ = val;
+        d1_.cat_ = &cat;
     }
 
     template<class ErrorCodeEnum> BOOST_SYSTEM_CONSTEXPR error_code( ErrorCodeEnum e,
@@ -63,13 +116,21 @@ public:
         *this = make_error_code( e );
     }
 
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+    error_code( std::error_code const& ec ) BOOST_NOEXCEPT:
+        flags_( 1 )
+    {
+        ::new( d2_ ) std::error_code( ec );
+    }
+
+#endif
+
     // modifiers:
 
     BOOST_SYSTEM_CONSTEXPR void assign( int val, const error_category & cat ) BOOST_NOEXCEPT
     {
-        val_ = val;
-        failed_ = detail::failed_impl( val, cat );
-        cat_ = &cat;
+        *this = error_code( val, cat );
     }
 
     template<typename ErrorCodeEnum>
@@ -82,48 +143,107 @@ public:
 
     BOOST_SYSTEM_CONSTEXPR void clear() BOOST_NOEXCEPT
     {
-        val_ = 0;
-        failed_ = false;
-        cat_ = &system_category();
+        *this = error_code();
     }
 
     // observers:
 
     BOOST_SYSTEM_CONSTEXPR int value() const BOOST_NOEXCEPT
     {
-        return val_;
+        if( flags_ != 1 )
+        {
+            return d1_.val_;
+        }
+        else
+        {
+            detail::std_error_code const& ec = reinterpret_cast<detail::std_error_code const&>( d2_ );
+            return ec.value() + static_cast<unsigned>( reinterpret_cast<boost::uintptr_t>( &ec.category() ) % 1073741789 /* 2^30-35, prime*/ );
+        }
     }
 
     BOOST_SYSTEM_CONSTEXPR const error_category & category() const BOOST_NOEXCEPT
     {
-        return *cat_;
+        if( flags_ == 0 )
+        {
+            return system_category();
+        }
+        else if( flags_ == 1 )
+        {
+            return detail::interop_category();
+        }
+        else
+        {
+            return *d1_.cat_;
+        }
     }
 
+    // deprecated?
     error_condition default_error_condition() const BOOST_NOEXCEPT
     {
-        return cat_->default_error_condition( value() );
+        return category().default_error_condition( value() );
     }
 
     std::string message() const
     {
-        return cat_->message( value() );
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( flags_ == 1 )
+        {
+            detail::std_error_code const& ec = reinterpret_cast<detail::std_error_code const&>( d2_ );
+            return ec.message();
+        }
+
+#endif
+
+        return category().message( value() );
     }
 
     char const * message( char * buffer, std::size_t len ) const BOOST_NOEXCEPT
     {
-        return cat_->message( value(), buffer, len );
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+        if( flags_ == 1 )
+        {
+#if !defined(BOOST_NO_EXCEPTIONS)
+            try
+#endif
+            {
+                detail::std_error_code const& ec = reinterpret_cast<detail::std_error_code const&>( d2_ );
+                detail::snprintf( buffer, len, "%s", ec.message().c_str() );
+                return buffer;
+            }
+#if !defined(BOOST_NO_EXCEPTIONS)
+            catch( ... )
+            {
+            }
+#endif
+        }
+#endif
+
+        return category().message( value(), buffer, len );
     }
 
     BOOST_SYSTEM_CONSTEXPR bool failed() const BOOST_NOEXCEPT
     {
-        return failed_;
+        if( flags_ == 0 )
+        {
+            return false;
+        }
+        else if( flags_ == 1 )
+        {
+            detail::std_error_code const& ec = reinterpret_cast<detail::std_error_code const&>( d2_ );
+            return ec.value() != 0;
+        }
+        else
+        {
+            return (flags_ & 1) != 0;
+        }
     }
 
 #if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
 
     BOOST_SYSTEM_CONSTEXPR explicit operator bool() const BOOST_NOEXCEPT  // true if error
     {
-        return failed_;
+        return failed();
     }
 
 #else
@@ -133,12 +253,12 @@ public:
 
     BOOST_SYSTEM_CONSTEXPR operator unspecified_bool_type() const  BOOST_NOEXCEPT // true if error
     {
-        return failed_? unspecified_bool_true: 0;
+        return failed()? unspecified_bool_true: 0;
     }
 
     BOOST_SYSTEM_CONSTEXPR bool operator!() const BOOST_NOEXCEPT // true if no error
     {
-        return !failed_;
+        return !failed();
     }
 
 #endif
@@ -150,37 +270,80 @@ public:
 
     BOOST_SYSTEM_CONSTEXPR inline friend bool operator==( const error_code & lhs, const error_code & rhs ) BOOST_NOEXCEPT
     {
-        return lhs.val_ == rhs.val_ && *lhs.cat_ == *rhs.cat_;
+        return lhs.value() == rhs.value() && lhs.category() == rhs.category();
     }
 
     BOOST_SYSTEM_CONSTEXPR inline friend bool operator<( const error_code & lhs, const error_code & rhs ) BOOST_NOEXCEPT
     {
-        return *lhs.cat_ < *rhs.cat_ || ( *lhs.cat_ == *rhs.cat_ && lhs.val_ < rhs.val_ );
+        return lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value());
+    }
+
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator!=( const error_code & lhs, const error_code & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
     }
 
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
     operator std::error_code () const
     {
-        return std::error_code( value(), category() );
+        if( flags_ == 1 )
+        {
+            return reinterpret_cast<std::error_code const&>( d2_ );
+        }
+        else if( flags_ == 0 )
+        {
+            //return std::error_code();
+            return std::error_code( 0, boost::system::system_category() );
+        }
+        else
+        {
+            return std::error_code( d1_.val_, *d1_.cat_ );
+        }
+    }
+
+    operator std::error_code ()
+    {
+        return const_cast<error_code const&>( *this );
+    }
+
+    template<class T,
+      class E = typename detail::enable_if<detail::is_same<T, std::error_code>::value>::type>
+      operator T& ()
+    {
+        if( flags_ != 1 )
+        {
+            std::error_code e2( *this );
+            ::new( d2_ ) std::error_code( e2 );
+            flags_ = 1;
+        }
+
+        return reinterpret_cast<std::error_code&>( d2_ );
     }
 
 #endif
+
+    template<class Ch, class Tr>
+        inline friend std::basic_ostream<Ch, Tr>&
+        operator<< (std::basic_ostream<Ch, Tr>& os, error_code const & ec)
+    {
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( ec.flags_ == 1 )
+        {
+            detail::std_error_code const& e2 = reinterpret_cast<detail::std_error_code const&>( ec.d2_ );
+            os << "std:" << e2.category().name() << ':' << e2.value();
+        }
+        else
+#endif
+        {
+            os << ec.category().name() << ':' << ec.value();
+        }
+
+        return os;
+    }
 };
 
-BOOST_SYSTEM_CONSTEXPR inline bool operator!=( const error_code & lhs, const error_code & rhs ) BOOST_NOEXCEPT
-{
-    return !( lhs == rhs );
-}
-
-template<class Ch, class Tr>
-    inline std::basic_ostream<Ch, Tr>&
-    operator<< (std::basic_ostream<Ch, Tr>& os, error_code const & ec)
-{
-    os << ec.category().name() << ':' << ec.value();
-    return os;
-}
-
 inline std::size_t hash_value( error_code const & ec )
 {
     error_category const & cat = ec.category();
diff --git a/include/boost/system/detail/interop_category.hpp b/include/boost/system/detail/interop_category.hpp
new file mode 100644
index 0000000..9755ad5
--- /dev/null
+++ b/include/boost/system/detail/interop_category.hpp
@@ -0,0 +1,107 @@
+#ifndef BOOST_SYSTEM_DETAIL_INTEROP_CATEGORY_HPP_INCLUDED
+#define BOOST_SYSTEM_DETAIL_INTEROP_CATEGORY_HPP_INCLUDED
+
+//  Copyright Beman Dawes 2006, 2007
+//  Copyright Christoper Kohlhoff 2007
+//  Copyright Peter Dimov 2017, 2018, 2021
+//
+//  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 library home page at http://www.boost.org/libs/system
+
+#include <boost/system/detail/error_category.hpp>
+#include <boost/system/detail/snprintf.hpp>
+#include <boost/system/detail/config.hpp>
+#include <boost/config.hpp>
+
+namespace boost
+{
+
+namespace system
+{
+
+namespace detail
+{
+
+// interop_error_category, used for std::error_code
+
+#if ( defined( BOOST_GCC ) && BOOST_GCC >= 40600 ) || defined( BOOST_CLANG )
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#endif
+
+class BOOST_SYMBOL_VISIBLE interop_error_category: public error_category
+{
+public:
+
+    BOOST_SYSTEM_CONSTEXPR interop_error_category() BOOST_NOEXCEPT:
+        error_category( detail::interop_category_id )
+    {
+    }
+
+    const char * name() const BOOST_NOEXCEPT BOOST_OVERRIDE
+    {
+        return "std:unknown";
+    }
+
+    std::string message( int ev ) const BOOST_OVERRIDE;
+    char const * message( int ev, char * buffer, std::size_t len ) const BOOST_NOEXCEPT BOOST_OVERRIDE;
+};
+
+#if ( defined( BOOST_GCC ) && BOOST_GCC >= 40600 ) || defined( BOOST_CLANG )
+#pragma GCC diagnostic pop
+#endif
+
+inline char const * interop_error_category::message( int ev, char * buffer, std::size_t len ) const BOOST_NOEXCEPT
+{
+    detail::snprintf( buffer, len, "Unknown interop error %d", ev );
+    return buffer;
+}
+
+inline std::string interop_error_category::message( int ev ) const
+{
+    char buffer[ 48 ];
+    return message( ev, buffer, sizeof( buffer ) );
+}
+
+// interop_category()
+
+#if defined(BOOST_SYSTEM_HAS_CONSTEXPR)
+
+template<class T> struct BOOST_SYMBOL_VISIBLE interop_cat_holder
+{
+    static constexpr interop_error_category instance{};
+};
+
+// Before C++17 it was mandatory to redeclare all static constexpr
+#if defined(BOOST_NO_CXX17_INLINE_VARIABLES)
+template<class T> constexpr interop_error_category interop_cat_holder<T>::instance;
+#endif
+
+constexpr error_category const & interop_category() BOOST_NOEXCEPT
+{
+    return interop_cat_holder<void>::instance;
+}
+
+#else // #if defined(BOOST_SYSTEM_HAS_CONSTEXPR)
+
+#if !defined(__SUNPRO_CC) // trailing __global is not supported
+inline error_category const & interop_category() BOOST_NOEXCEPT BOOST_SYMBOL_VISIBLE;
+#endif
+
+inline error_category const & interop_category() BOOST_NOEXCEPT
+{
+    static const detail::interop_error_category instance;
+    return instance;
+}
+
+#endif // #if defined(BOOST_SYSTEM_HAS_CONSTEXPR)
+
+} // namespace detail
+
+} // namespace system
+
+} // namespace boost
+
+#endif // #ifndef BOOST_SYSTEM_DETAIL_INTEROP_CATEGORY_HPP_INCLUDED
diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4dd3ab8..2d29e40 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -89,3 +89,7 @@ boost_test(TYPE run SOURCES linux_error_test.cpp)
 boost_test(TYPE link SOURCES errc_test3.cpp)
 
 boost_test(TYPE run SOURCES snprintf_test.cpp)
+
+boost_test(TYPE run SOURCES std_interop_test2.cpp)
+boost_test(TYPE run SOURCES std_interop_test3.cpp)
+boost_test(TYPE run SOURCES std_interop_test4.cpp)
diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
index 3061abe..9940d36 100644
--- a/test/Jamfile.v2
+++ b/test/Jamfile.v2
@@ -93,3 +93,7 @@ run linux_error_test.cpp ;
 link errc_test3.cpp ;
 
 run snprintf_test.cpp ;
+
+run std_interop_test2.cpp ;
+run std_interop_test3.cpp ;
+run std_interop_test4.cpp ;
diff --git a/test/std_interop_test2.cpp b/test/std_interop_test2.cpp
new file mode 100644
index 0000000..a1dccd0
--- /dev/null
+++ b/test/std_interop_test2.cpp
@@ -0,0 +1,88 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/system/system_category.hpp>
+#include <boost/system/generic_category.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cerrno>
+
+#if !defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+BOOST_PRAGMA_MESSAGE( "BOOST_SYSTEM_HAS_SYSTEM_ERROR not defined, test will be skipped" )
+int main() {}
+
+#else
+
+#include <system_error>
+
+void f1( std::error_code ec, int value, std::error_category const& category )
+{
+    BOOST_TEST_EQ( ec.value(), value );
+    BOOST_TEST_EQ( &ec.category(), &category );
+}
+
+void f2( std::error_code const& ec, int value, std::error_category const& category )
+{
+    BOOST_TEST_EQ( ec.value(), value );
+    BOOST_TEST_EQ( &ec.category(), &category );
+}
+
+int main()
+{
+    {
+        boost::system::error_code e1;
+        boost::system::error_code e2( e1 );
+
+        f1( e1, e1.value(), e1.category() );
+        f2( e1, e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    {
+        boost::system::error_code e1( 0, boost::system::system_category() );
+        boost::system::error_code e2( e1 );
+
+        f1( e1, e1.value(), e1.category() );
+        f2( e1, e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    {
+        boost::system::error_code e1( 5, boost::system::system_category() );
+        boost::system::error_code e2( e1 );
+
+        f1( e1, e1.value(), e1.category() );
+        f2( e1, e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    {
+        boost::system::error_code e1( 0, boost::system::generic_category() );
+        boost::system::error_code e2( e1 );
+
+        f1( e1, e1.value(), e1.category() );
+        f2( e1, e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    {
+        boost::system::error_code e1( ENOENT, boost::system::generic_category() );
+        boost::system::error_code e2( e1 );
+
+        f1( e1, e1.value(), e1.category() );
+        f2( e1, e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif
diff --git a/test/std_interop_test3.cpp b/test/std_interop_test3.cpp
new file mode 100644
index 0000000..9584bd7
--- /dev/null
+++ b/test/std_interop_test3.cpp
@@ -0,0 +1,63 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cerrno>
+
+#if !defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+BOOST_PRAGMA_MESSAGE( "BOOST_SYSTEM_HAS_SYSTEM_ERROR not defined, test will be skipped" )
+int main() {}
+
+#else
+
+#include <system_error>
+
+int main()
+{
+    {
+        std::error_code e1;
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 5, std::system_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 0, std::generic_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( ENOENT, std::generic_category() );
+        boost::system::error_code e2 = e1;
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        boost::system::error_code e2 = make_error_code( std::errc::no_such_file_or_directory );
+        std::error_code e3 = e2;
+
+        BOOST_TEST_EQ( make_error_code( std::errc::no_such_file_or_directory ), e3 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif
diff --git a/test/std_interop_test4.cpp b/test/std_interop_test4.cpp
new file mode 100644
index 0000000..ab74e3f
--- /dev/null
+++ b/test/std_interop_test4.cpp
@@ -0,0 +1,89 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <cerrno>
+
+#if !defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+BOOST_PRAGMA_MESSAGE( "BOOST_SYSTEM_HAS_SYSTEM_ERROR not defined, test will be skipped" )
+int main() {}
+
+#else
+
+#include <system_error>
+
+void f( std::error_code& e1, std::error_code e2 )
+{
+    e1 = e2;
+}
+
+int main()
+{
+    {
+        boost::system::error_code e1;
+        std::error_code e2;
+
+        f( e1, e2 );
+
+        BOOST_TEST_EQ( e1, boost::system::error_code( e2 ) );
+
+        std::error_code e3( e1 );
+        BOOST_TEST_EQ( e3, e2 );
+    }
+
+    {
+        boost::system::error_code e1;
+        std::error_code e2( 0, std::system_category() );
+
+        f( e1, e2 );
+
+        BOOST_TEST_EQ( e1, boost::system::error_code( e2 ) );
+
+        std::error_code e3( e1 );
+        BOOST_TEST_EQ( e3, e2 );
+    }
+
+    {
+        boost::system::error_code e1;
+        std::error_code e2( 5, std::system_category() );
+
+        f( e1, e2 );
+
+        BOOST_TEST_EQ( e1, boost::system::error_code( e2 ) );
+
+        std::error_code e3( e1 );
+        BOOST_TEST_EQ( e3, e2 );
+    }
+
+    {
+        boost::system::error_code e1;
+        std::error_code e2( 0, std::generic_category() );
+
+        f( e1, e2 );
+
+        BOOST_TEST_EQ( e1, boost::system::error_code( e2 ) );
+
+        std::error_code e3( e1 );
+        BOOST_TEST_EQ( e3, e2 );
+    }
+
+    {
+        boost::system::error_code e1;
+        std::error_code e2( ENOENT, std::generic_category() );
+
+        f( e1, e2 );
+
+        BOOST_TEST_EQ( e1, boost::system::error_code( e2 ) );
+
+        std::error_code e3( e1 );
+        BOOST_TEST_EQ( e3, e2 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif