Update documentation

This reverts commit c069ae048f.

.travis.yml

@@ -65,14 +65,19 @@ matrix:
           sources:
             - ubuntu-toolchain-r-test
 
+    - os: linux
+      dist: trusty
+      compiler: g++
+      env: TOOLSET=gcc COMPILER=g++ CXXSTD=03,11
+
     - os: linux
       dist: bionic
-      compiler: g++-10
-      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-10 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
+      compiler: g++-11
+      env: UBSAN=1 TOOLSET=gcc COMPILER=g++-11 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1 LINKFLAGS=-fuse-ld=gold
       addons:
         apt:
           packages:
-            - g++-10
+            - g++-11
           sources:
             - sourceline: "ppa:ubuntu-toolchain-r/test"
 
@@ -95,15 +100,15 @@ matrix:
             - clang-3.4
 
     - os: linux
-      compiler: clang++-11
-      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-11 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1
+      compiler: clang++-12
+      env: UBSAN=1 TOOLSET=clang COMPILER=clang++-12 CXXSTD=03,11,14,17,20 UBSAN_OPTIONS=print_stacktrace=1
       addons:
         apt:
           packages:
-            - clang-11
+            - clang-12
           sources:
             - ubuntu-toolchain-r-test
-            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-11 main'
+            - sourceline: 'deb https://apt.llvm.org/xenial/ llvm-toolchain-xenial-12 main'
               key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - os: linux
@@ -206,7 +211,7 @@ install:
 script:
   - |-
     echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
-  - ./b2 -j3 libs/system/test toolset=$TOOLSET cxxstd=$CXXSTD variant=debug,release ${ADDRMD:+address-model=$ADDRMD} ${UBSAN:+undefined-sanitizer=norecover define=UBSAN=1 debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}
+  - ./b2 -j3 libs/system/test toolset=$TOOLSET cxxstd=$CXXSTD variant=debug,release ${ADDRMD:+address-model=$ADDRMD} ${UBSAN:+cxxflags=-fsanitize=undefined cxxflags=-fno-sanitize-recover=undefined linkflags=-fsanitize=undefined define=UBSAN=1 debug-symbols=on} ${LINKFLAGS:+linkflags=$LINKFLAGS}
 
 notifications:
   email:

appveyor.yml

@@ -30,8 +30,12 @@ environment:
       ADDRMD: 64
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: msvc-14.2
-      CXXSTD: 14,17
+      CXXSTD: 14,17,latest
       ADDRMD: 32,64
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      TOOLSET: clang-win
+      CXXSTD: 14,17,latest
+      ADDRMD: 64
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       ADDPATH: C:\cygwin\bin;
       TOOLSET: gcc
@@ -67,4 +71,4 @@ test_script:
   - PATH=%ADDPATH%%PATH%
   - if not "%CXXSTD%" == "" set CXXSTD=cxxstd=%CXXSTD%
   - if not "%ADDRMD%" == "" set ADDRMD=address-model=%ADDRMD%
-  - b2 -j3 libs/system/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release
+  - b2 -j3 libs/system/test toolset=%TOOLSET% %CXXSTD% %ADDRMD% variant=debug,release embed-manifest-via=linker

doc/system/changes.adoc

@@ -27,6 +27,11 @@ http://www.boost.org/LICENSE_1_0.txt
   an `error_condition` from the generic category, instead of from the system
   category. This happens on POSIX when the input `error_code` is from
   the system category and does not correspond to any `errc_t` value.
+* The interoperability of `error_code` and `std::error_code` has been
+  improved substantially. It is now possible to construct
+  `boost::system::error_code` from `std::error_code`, and it's possible
+  to pass `boost::system::error_code` to functions taking `std::error_code&`.
+* A stream insertion operator for `error_condition` has been added.
 
 ## Changes in Boost 1.76
 

doc/system/reference.adoc

@@ -567,6 +567,8 @@ namespace boost {
       template <class ErrorCodeEnum>
         constexpr error_code( ErrorCodeEnum e ) noexcept;
 
+      error_code( std::error_code const& ec ) noexcept;
+
       // modifiers:
 
       constexpr void assign( int val, const error_category & cat ) noexcept;
@@ -593,33 +595,58 @@ namespace boost {
 
       friend constexpr bool operator==( const error_code & lhs,
         const error_code & rhs ) noexcept;
+
       friend constexpr bool operator!=( const error_code & lhs,
         const error_code & rhs ) noexcept;
 
       friend constexpr bool operator<( const error_code & lhs,
         const error_code & rhs ) noexcept;
 
+      friend bool operator==( const error_code & code,
+        const error_condition & condition ) noexcept;
+      friend bool operator==( const error_condition & condition,
+        const error_code & code ) noexcept;
+
+      friend bool operator!=( const error_code & code,
+        const error_condition & condition ) noexcept;
+      friend bool operator!=( const error_condition & condition,
+        const error_code & code ) noexcept;
+
+      friend bool operator==( const error_code & lhs,
+        const std::error_code & rhs ) noexcept;
+      friend bool operator==( const std::error_code & lhs,
+        const error_code & rhs ) noexcept;
+
+      friend bool operator!=( const error_code & lhs,
+        const std::error_code & rhs ) noexcept;
+      friend bool operator!=( const std::error_code & lhs,
+        const error_code & rhs ) noexcept;
+
+      template<class E>
+      friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
+      template<class E>
+      friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
+
+      template<class E>
+      friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
+      template<class E>
+      friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;
+
       // conversions:
 
       operator std::error_code() const;
+      operator std::error_code();
+      template<class T> operator T& (); // only when T=std::error_code
+
+      // stream insertion:
+
+      template <class charT, class traits>
+        std::basic_ostream<charT, traits>&
+          operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
     };
 
     // non-member functions
 
-    bool operator==( const error_code & code,
-      const error_condition & condition ) noexcept;
-    bool operator==( const error_condition & condition,
-      const error_code & code ) noexcept;
-
-    bool operator!=( const error_code & code,
-      const error_condition & condition ) noexcept;
-    bool operator!=( const error_condition & condition,
-      const error_code & code ) noexcept;
-
-    template <class charT, class traits>
-      std::basic_ostream<charT, traits>&
-        operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
-
     std::size_t hash_value( const error_code & ec );
 
   }
@@ -654,6 +681,15 @@ template <class ErrorCodeEnum>
 Ensures: :: `*this == make_error_code( e )`.
 Remarks: :: This constructor is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.
 
+```
+error_code( std::error_code const & ec ) noexcept;
+```
+[none]
+* {blank}
++
+Effects: :: Construct an `error_code` that wraps `ec`.
+Remarks: :: `value()` and `category()` are unspecified.
+
 #### Modifiers
 
 ```
@@ -715,7 +751,8 @@ std::string message() const;
 [none]
 * {blank}
 +
-Returns: :: `category().message( value() )`.
+Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec.message()`.
+  Otherwise, `category().message( value() )`.
 
 ```
 char const * message( char * buffer, std::size_t len ) const noexcept;
@@ -723,7 +760,9 @@ char const * message( char * buffer, std::size_t len ) const noexcept;
 [none]
 * {blank}
 +
-Returns: :: `category().message( value(), buffer, len )`.
+Effects: :: If `*this` wraps a `std::error_code` object `ec`, copies the
+  string returned from `ec.message()` into `buffer` and returns `buffer`.
+  Otherwise, returns `category().message( value(), buffer, len )`.
 
 ```
 constexpr bool failed() const noexcept;
@@ -731,7 +770,8 @@ constexpr bool failed() const noexcept;
 [none]
 * {blank}
 +
-Returns: :: `category().failed( value() )`.
+Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec.value() != 0`.
+  Otherwise, `category().failed( value() )`.
 
 ```
 constexpr explicit operator bool() const noexcept;
@@ -751,7 +791,8 @@ friend constexpr bool operator==( const error_code & lhs,
 [none]
 * {blank}
 +
-Returns: :: `lhs.value() == rhs.value() && lhs.category() == rhs.category()`.
+Returns: :: If both `lhs` and `rhs` wrap `std::error_code` objects `e1` and `e2`, `e1 == e2`.
+  Otherwise, `lhs.value() == rhs.value() && lhs.category() == rhs.category()`.
 
 ```
 friend constexpr bool operator!=( const error_code & lhs,
@@ -769,43 +810,138 @@ friend constexpr bool operator<( const error_code & lhs,
 [none]
 * {blank}
 +
+Returns: :: If both `lhs` and `rhs` wrap `std::error_code` objects `e1` and `e2`, `e1 < e2`.
+  Otherwise, `lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())`.
+
+```
+friend bool operator==( const error_code & code,
+  const error_condition & condition ) noexcept;
+```
+```
+friend bool operator==( const error_condition & condition,
+  const error_code & code ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: If `code` wraps a `std::error_code` object `ec`, `ec == static_cast<std::error_condition>( condition )`.
+  Otherwise, `code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() )`.
+
+```
+friend bool operator!=( const error_code & lhs,
+  const error_condition & rhs ) noexcept;
+```
+```
+friend bool operator!=( const error_condition & lhs,
+  const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `!( lhs == rhs )`.
+
+```
+friend bool operator==( const error_code & lhs,
+  const std::error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `static_cast<std::error_code>(lhs) == rhs`.
+
+```
+friend bool operator==( const std::error_code & lhs,
+  const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `lhs == static_cast<std::error_code>(rhs)`.
+
+```
+friend bool operator!=( const error_code & lhs,
+  const std::error_code & rhs ) noexcept;
+```
+```
+friend bool operator!=( const std::error_code & lhs,
+  const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
 Returns: ::
-  `lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())`.
+  `!( lhs == rhs )`.
+
+```
+template<class E>
+friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Effects: ::
+[disc]
+** When `is_error_code_enum<E>::value` is `true`, returns `lhs == make_error_code(rhs)`;
+** When `is_error_condition_enum<E>::value` is `true`, returns `lhs == make_error_condition(rhs)`;
+** Otherwise, this overload is disabled.
+
+```
+template<class E>
+friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Effects: ::
+[disc]
+** When `is_error_code_enum<E>::value` is `true`, returns `make_error_code(lhs) == rhs`;
+** When `is_error_condition_enum<E>::value` is `true`, returns `make_error_condition(lhs) == rhs`;
+** Otherwise, this overload is disabled.
+
+```
+template<class E>
+friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
+```
+```
+template<class E>
+friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `!( lhs == rhs )`.
+Remarks: :: These overloads are only enabled when
+  `is_error_code_enum<E>::value` is `true` or
+  `is_error_condition_enum<E>::value` is `true`.
 
 #### Conversions
 
 ```
 operator std::error_code() const;
 ```
-[none]
-* {blank}
-+
-Returns: ::
-  `std::error_code( value(), category() )`.
-
-#### Nonmembers
-
 ```
-bool operator==( const error_code & code,
-  const error_condition & condition ) noexcept;
-bool operator==( const error_condition & condition,
-  const error_code & code ) noexcept;
+operator std::error_code();
 ```
 [none]
 * {blank}
 +
-Returns: :: `code.category().equivalent(code.value(), condition) || condition.category().equivalent(code, condition.value())`.
+Returns: :: If `*this` wraps a `std::error_code` object `ec`, `ec`.
+  Otherwise, `std::error_code( value(), category() )`.
 
 ```
-bool operator!=( const error_code & code,
-  const error_condition & condition ) noexcept;
-bool operator!=( const error_condition & condition,
-  const error_code & code ) noexcept;
+template<class T> operator T&();
 ```
 [none]
 * {blank}
 +
-Returns: :: `!( lhs == rhs )`.
+Effects: :: If `*this` wraps a `std::error_code` object `ec`,
+  returns a reference to `ec`.
+  Otherwise, makes `*this` wrap `std::error_code( *this )`,
+  then returns a reference to it.
+Remarks: ::
+  This operator is only enabled when `T` is `std::error_code`.
+
+#### Stream Insertion
 
 ```
 template <class charT, class traits>
@@ -815,8 +951,12 @@ template <class charT, class traits>
 [none]
 * {blank}
 +
-Effects: :: `os << ec.category().name() << ':' << ec.value()`.
-Returns: :: `os`.
+Effects: :: If `ec` wraps a `std::error_code` object `e2`, `os << "std:" << e2`.
+  Otherwise, `os << ec.category().name() << ':' << ec.value()`.
+Returns: ::
+  `os`.
+
+#### Nonmembers
 
 ```
 std::size_t hash_value( const error_code & ec );
@@ -824,8 +964,8 @@ std::size_t hash_value( const error_code & ec );
 [none]
 * {blank}
 +
-Returns: ::
-  A hash value representing `ec`.
+Returns: :: If `ec` wraps a `std::error_code` object `e2`, `std::hash<std::error_code>()(e2)`.
+  Otherwise, a hash value representing `ec`.
 
 ## <boost/system/{zwsp}error_condition.hpp>
 
@@ -870,15 +1010,32 @@ namespace boost {
 
       friend constexpr bool operator==( const error_condition & lhs,
         const error_condition & rhs ) noexcept;
+
       friend constexpr bool operator!=( const error_condition & lhs,
         const error_condition & rhs ) noexcept;
 
       friend constexpr bool operator<( const error_condition & lhs,
         const error_condition & rhs ) noexcept;
 
+      friend bool operator==( const std::error_code & code,
+        const error_condition & condition ) noexcept;
+      friend bool operator==( const error_condition & condition,
+        const std::error_code & code ) noexcept;
+
+      friend bool operator!=( const std::error_code & code,
+        const error_condition & condition ) noexcept;
+      friend bool operator!=( const error_condition & condition,
+        const std::error_code & code ) noexcept;
+
       // conversions:
 
       operator std::error_condition() const;
+
+      // stream insertion:
+
+      template <class charT, class traits>
+        std::basic_ostream<charT, traits>&
+          operator<<( basic_ostream<charT, traits>& os, const error_condition & en );
     };
   }
 }
@@ -1023,6 +1180,33 @@ friend constexpr bool operator<( const error_condition & lhs,
 Returns: ::
   `lhs.category() < rhs.category() || (lhs.category() == rhs.category() && lhs.value() < rhs.value())`.
 
+```
+friend bool operator==( const std::error_code & code,
+  const error_condition & condition ) noexcept;
+```
+```
+friend bool operator==( const error_condition & condition,
+  const std::error_code & code ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `code == static_cast<std::error_condition>( rhs )`.
+
+```
+friend constexpr bool operator!=( const std::error_code & lhs,
+  const error_condition & rhs ) noexcept;
+```
+```
+friend constexpr bool operator!=( const error_condition & lhs,
+  const std::error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: ::
+  `!( lhs == rhs )`.
+
 #### Conversions
 
 ```
@@ -1034,6 +1218,20 @@ operator std::error_condition() const;
 Returns: ::
   `std::error_condition( value(), category() )`.
 
+#### Stream Insertion
+
+```
+template <class charT, class traits>
+  std::basic_ostream<charT, traits>&
+    operator<<( basic_ostream<charT, traits>& os, const error_condition & en );
+```
+[none]
+* {blank}
++
+Effects: :: `os << "cond:" << en.category().name() << ':' << en.value()`.
+Returns: ::
+  `os`.
+
 ## <boost/system/{zwsp}system_error.hpp>
 
 ### Class system_error

include/boost/system/detail/config.hpp

@@ -56,4 +56,10 @@
 # define BOOST_SYSTEM_DEPRECATED(msg)
 #endif
 
+// BOOST_SYSTEM_CLANG_6
+
+#if defined(__clang__) && (__clang_major__ < 7 || (defined(__APPLE__) && __clang_major__ < 11))
+# define BOOST_SYSTEM_CLANG_6
+#endif
+
 #endif // BOOST_SYSTEM_DETAIL_CONFIG_HPP_INCLUDED

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 )
 {

include/boost/system/detail/error_category_impl.hpp

@@ -13,6 +13,7 @@
 #include <boost/system/detail/error_category.hpp>
 #include <boost/system/detail/error_condition.hpp>
 #include <boost/system/detail/error_code.hpp>
+#include <boost/system/detail/snprintf.hpp>
 #include <boost/config.hpp>
 #include <string>
 #include <cstring>
@@ -83,7 +84,8 @@ inline char const * error_category::message( int ev, char * buffer, std::size_t
 #if !defined(BOOST_NO_EXCEPTIONS)
     catch( ... )
     {
-        return "Message text unavailable";
+        detail::snprintf( buffer, len, "No message text available for error %d", ev );
+        return buffer;
     }
 #endif
 }

include/boost/system/detail/error_code.hpp

@@ -10,15 +10,29 @@
 //
 //  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
+
+#if defined(BOOST_GCC) && BOOST_GCC >= 40600 && BOOST_GCC < 70000
+# pragma GCC diagnostic push
+# pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
 
 namespace boost
 {
@@ -35,26 +49,50 @@ namespace system
 //  and error_code containing a pointer to an object of a type derived
 //  from error_category.
 
+std::size_t hash_value( error_code const & ec );
+
 class error_code
 {
 private:
 
-    int val_;
-    bool failed_;
-    const error_category * cat_;
+    friend std::size_t hash_value( error_code const & ec );
+
+private:
+
+    struct data
+    {
+        int val_;
+        const error_category * cat_;
+    };
+
+    union
+    {
+        data d1_;
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+        unsigned char d2_[ sizeof(std::error_code) ];
+#endif
+    };
+
+    // 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 +101,21 @@ 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 +128,113 @@ 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
+        {
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+            std::error_code const& ec = *reinterpret_cast<std::error_code const*>( d2_ );
+            return ec.value() + 1000 * static_cast<unsigned>( reinterpret_cast<boost::uintptr_t>( &ec.category() ) % 2097143 ); // 2^21-9, prime
+#else
+
+            return -1;
+#endif
+        }
     }
 
     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 )
+        {
+            std::error_code const& ec = *reinterpret_cast<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
+            {
+                std::error_code const& ec = *reinterpret_cast<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 defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( flags_ == 1 )
+        {
+            std::error_code const& ec = *reinterpret_cast<std::error_code const*>( d2_ );
+            return ec.value() != 0;
+        }
+        else
+
+#endif
+        {
+            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 +244,12 @@ 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
@@ -150,39 +261,266 @@ 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_;
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( lhs.flags_ == 1 && rhs.flags_ == 1 )
+        {
+            std::error_code const& e1 = *reinterpret_cast<std::error_code const*>( lhs.d2_ );
+            std::error_code const& e2 = *reinterpret_cast<std::error_code const*>( rhs.d2_ );
+
+            return e1 == e2;
+        }
+        else
+#endif
+        {
+            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_ );
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( lhs.flags_ == 1 && rhs.flags_ == 1 )
+        {
+            std::error_code const& e1 = *reinterpret_cast<std::error_code const*>( lhs.d2_ );
+            std::error_code const& e2 = *reinterpret_cast<std::error_code const*>( rhs.d2_ );
+
+            return e1 < e2;
+        }
+        else
+#endif
+        {
+            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 );
+    }
+
+    inline friend bool operator==( const error_code & code, const error_condition & condition ) BOOST_NOEXCEPT
+    {
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( code.flags_ == 1 )
+        {
+            return static_cast<std::error_code>( code ) == static_cast<std::error_condition>( condition );
+        }
+        else
+
+#endif
+        {
+            return code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() );
+        }
+    }
+
+    inline friend bool operator==( const error_condition & condition, const error_code & code ) BOOST_NOEXCEPT
+    {
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+        if( code.flags_ == 1 )
+        {
+            return static_cast<std::error_code>( code ) == static_cast<std::error_condition>( condition );
+        }
+        else
+
+#endif
+        {
+            return code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() );
+        }
+    }
+
+    inline friend bool operator!=( const error_code & lhs, const error_condition & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    inline friend bool operator!=( const error_condition & lhs, const error_code & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
     }
 
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
-    operator std::error_code () const
+    inline friend bool operator==( std::error_code const & lhs, error_code const & rhs ) BOOST_NOEXCEPT
     {
-        return std::error_code( value(), category() );
+        return lhs == static_cast< std::error_code >( rhs );
+    }
+
+    inline friend bool operator==( error_code const & lhs, std::error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return static_cast< std::error_code >( lhs ) == rhs;
+    }
+
+    inline friend bool operator!=( std::error_code const & lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    inline friend bool operator!=( error_code const & lhs, std::error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    //
+
+    template<class E, class N = typename detail::enable_if<std::is_error_condition_enum<E>::value>::type>
+    inline friend bool operator==( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    {
+        return lhs == make_error_condition( rhs );
+    }
+
+    template<class E, class N = typename detail::enable_if<std::is_error_condition_enum<E>::value>::type>
+    inline friend bool operator==( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return make_error_condition( lhs ) == rhs;
+    }
+
+    template<class E, class N = typename detail::enable_if<std::is_error_condition_enum<E>::value>::type>
+    inline friend bool operator!=( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    template<class E, class N = typename detail::enable_if<std::is_error_condition_enum<E>::value>::type>
+    inline friend bool operator!=( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    //
+
+    template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator==( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    {
+        return lhs == make_error_code( rhs );
+    }
+
+    template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator==( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return make_error_code( lhs ) == rhs;
+    }
+
+    template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator!=( error_code const & lhs, E rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    template<class E, class N1 = void, class N2 = typename detail::enable_if<std::is_error_code_enum<E>::value>::type>
+    BOOST_SYSTEM_CONSTEXPR inline friend bool operator!=( E lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+#if defined(BOOST_SYSTEM_CLANG_6)
+
+    inline friend bool operator==( error_code const & lhs, std::error_condition const & rhs ) BOOST_NOEXCEPT
+    {
+        return static_cast< std::error_code >( lhs ) == rhs;
+    }
+
+    inline friend bool operator==( std::error_condition const & lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return lhs == static_cast< std::error_code >( rhs );
+    }
+
+    inline friend bool operator!=( error_code const & lhs, std::error_condition const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    inline friend bool operator!=( std::error_condition const & lhs, error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
     }
 
 #endif
+
+    // conversions
+
+    operator std::error_code () const
+    {
+        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_ );
+    }
+
+#if defined(BOOST_SYSTEM_CLANG_6)
+
+    template<class T,
+      class E = typename std::enable_if<std::is_same<T, std::error_code>::value>::type>
+      operator T const& () = delete;
+
+#endif
+
+#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 )
+        {
+            std::error_code const& e2 = *reinterpret_cast<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 )
 {
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+    if( ec.flags_ == 1 )
+    {
+        std::error_code const& e2 = *reinterpret_cast<std::error_code const*>( ec.d2_ );
+        return std::hash<std::error_code>()( e2 );
+    }
+
+#endif
+
     error_category const & cat = ec.category();
 
     boost::ulong_long_type id_ = cat.id_;
@@ -212,4 +550,8 @@ inline std::size_t hash_value( error_code const & ec )
 
 } // namespace boost
 
+#if defined(BOOST_GCC) && BOOST_GCC >= 40600 && BOOST_GCC < 70000
+# pragma GCC diagnostic pop
+#endif
+
 #endif // #ifndef BOOST_SYSTEM_DETAIL_ERROR_CODE_HPP_INCLUDED

include/boost/system/detail/error_condition.hpp

@@ -45,6 +45,7 @@ class error_condition
 private:
 
     int val_;
+    bool failed_;
     error_category const * cat_;
 
 public:
@@ -52,17 +53,17 @@ public:
     // constructors:
 
     BOOST_SYSTEM_CONSTEXPR error_condition() BOOST_NOEXCEPT:
-        val_( 0 ), cat_( 0 )
+        val_( 0 ), failed_( false ), cat_( 0 )
     {
     }
 
     BOOST_SYSTEM_CONSTEXPR error_condition( int val, const error_category & cat ) BOOST_NOEXCEPT:
-        val_( val ), cat_( &cat )
+        val_( val ), failed_( detail::failed_impl( val, cat ) ), cat_( &cat )
     {
     }
 
     BOOST_SYSTEM_CONSTEXPR explicit error_condition( boost::system::detail::generic_value_tag vt ) BOOST_NOEXCEPT:
-        val_( vt.value ), cat_( 0 )
+        val_( vt.value ), failed_( vt.value != 0 ), cat_( 0 )
     {
     }
 
@@ -76,7 +77,7 @@ public:
 
     template<class ErrorConditionEnum> BOOST_SYSTEM_CONSTEXPR error_condition( ErrorConditionEnum e,
       typename detail::enable_if<boost::system::detail::is_same<ErrorConditionEnum, errc::errc_t>::value>::type* = 0) BOOST_NOEXCEPT:
-        val_( e ), cat_( 0 )
+        val_( e ), failed_( e != 0 ), cat_( 0 )
     {
     }
 
@@ -85,6 +86,7 @@ public:
     BOOST_SYSTEM_CONSTEXPR void assign( int val, const error_category & cat ) BOOST_NOEXCEPT
     {
         val_ = val;
+        failed_ = detail::failed_impl( val, cat );
         cat_ = &cat;
     }
 
@@ -99,6 +101,7 @@ public:
     BOOST_SYSTEM_CONSTEXPR void clear() BOOST_NOEXCEPT
     {
         val_ = 0;
+        failed_ = false;
         cat_ = 0;
     }
 
@@ -140,21 +143,14 @@ public:
 
     BOOST_SYSTEM_CONSTEXPR bool failed() const BOOST_NOEXCEPT
     {
-        if( cat_ )
-        {
-            return detail::failed_impl( val_, *cat_ );
-        }
-        else
-        {
-            return val_ != 0;
-        }
+        return failed_;
     }
 
 #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
@@ -164,12 +160,12 @@ 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
@@ -202,7 +198,38 @@ public:
         return std::error_condition( value(), category() );
     }
 
+    inline friend bool operator==( std::error_code const & lhs, error_condition const & rhs ) BOOST_NOEXCEPT
+    {
+        return lhs == static_cast< std::error_condition >( rhs );
+    }
+
+    inline friend bool operator==( error_condition const & lhs, std::error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return static_cast< std::error_condition >( lhs ) == rhs;
+    }
+
+    inline friend bool operator!=( std::error_code const & lhs, error_condition const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
+    inline friend bool operator!=( error_condition const & lhs, std::error_code const & rhs ) BOOST_NOEXCEPT
+    {
+        return !( lhs == rhs );
+    }
+
 #endif
+
+    template<class Ch, class Tr>
+        inline friend std::basic_ostream<Ch, Tr>&
+        operator<< (std::basic_ostream<Ch, Tr>& os, error_condition const & en)
+    {
+        {
+            os << "cond:" << en.category().name() << ':' << en.value();
+        }
+
+        return os;
+    }
 };
 
 } // namespace system

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

include/boost/system/detail/snprintf.hpp

@@ -0,0 +1,70 @@
+#ifndef BOOST_SYSTEM_DETAIL_SNPRINTF_HPP_INCLUDED
+#define BOOST_SYSTEM_DETAIL_SNPRINTF_HPP_INCLUDED
+
+// Copyright 2018, 2020, 2021 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 library home page at http://www.boost.org/libs/system
+
+#include <boost/config.hpp>
+#include <cstdio>
+#include <cstdarg>
+
+//
+
+namespace boost
+{
+
+namespace system
+{
+
+namespace detail
+{
+
+#if ( defined(_MSC_VER) && _MSC_VER < 1900 ) || ( defined(__MINGW32__) && !defined(__MINGW64_VERSION_MAJOR) )
+
+inline void snprintf( char * buffer, std::size_t len, char const * format, ... )
+{
+# if defined( BOOST_MSVC )
+#  pragma warning( push )
+#  pragma warning( disable: 4996 )
+# endif
+
+    if( len == 0 ) return;
+
+    va_list args;
+    va_start( args, format );
+
+    _vsnprintf( buffer, len - 1, format, args );
+    buffer[ len - 1 ] = 0;
+
+    va_end( args );
+
+# if defined( BOOST_MSVC )
+#  pragma warning( pop )
+# endif
+}
+
+#else
+
+inline void snprintf( char * buffer, std::size_t len, char const * format, ... )
+{
+    va_list args;
+    va_start( args, format );
+
+    std::vsnprintf( buffer, len, format, args );
+
+    va_end( args );
+}
+
+#endif
+
+} // namespace detail
+
+} // namespace system
+
+} // namespace boost
+
+#endif // #ifndef BOOST_SYSTEM_DETAIL_SNPRINTF_HPP_INCLUDED

include/boost/system/detail/system_category_message_win32.hpp

@@ -10,6 +10,7 @@
 //
 // See library home page at http://www.boost.org/libs/system
 
+#include <boost/system/detail/snprintf.hpp>
 #include <boost/winapi/error_handling.hpp>
 #include <boost/winapi/character_code_conversion.hpp>
 #include <boost/winapi/local_memory.hpp>
@@ -27,35 +28,12 @@ namespace system
 namespace detail
 {
 
-#if ( defined(_MSC_VER) && _MSC_VER < 1900 ) || ( defined(__MINGW32__) && !defined(__MINGW64_VERSION_MAJOR) )
-
 inline char const * unknown_message_win32( int ev, char * buffer, std::size_t len )
 {
-# if defined( BOOST_MSVC )
-#  pragma warning( push )
-#  pragma warning( disable: 4996 )
-# endif
-
-    _snprintf( buffer, len - 1, "Unknown error (%d)", ev );
-
-    buffer[ len - 1 ] = 0;
-    return buffer;
-
-# if defined( BOOST_MSVC )
-#  pragma warning( pop )
-# endif
-}
-
-#else
-
-inline char const * unknown_message_win32( int ev, char * buffer, std::size_t len )
-{
-    std::snprintf( buffer, len, "Unknown error (%d)", ev );
+    detail::snprintf( buffer, len, "Unknown error (%d)", ev );
     return buffer;
 }
 
-#endif
-
 inline boost::winapi::UINT_ message_cp_win32()
 {
 #if defined(BOOST_SYSTEM_USE_UTF8)

include/boost/system/error_code.hpp

@@ -17,38 +17,5 @@
 #include <boost/system/generic_category.hpp>
 #include <boost/system/system_category.hpp>
 #include <boost/system/detail/throws.hpp>
-#include <boost/config.hpp>
-
-namespace boost
-{
-
-namespace system
-{
-
-// non-member functions of error_code and error_condition
-
-inline bool operator==( const error_code & code, const error_condition & condition ) BOOST_NOEXCEPT
-{
-    return code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() );
-}
-
-inline bool operator!=( const error_code & lhs, const error_condition & rhs ) BOOST_NOEXCEPT
-{
-    return !( lhs == rhs );
-}
-
-inline bool operator==( const error_condition & condition, const error_code & code ) BOOST_NOEXCEPT
-{
-    return code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() );
-}
-
-inline bool operator!=( const error_condition & lhs, const error_code & rhs ) BOOST_NOEXCEPT
-{
-    return !( lhs == rhs );
-}
-
-} // namespace system
-
-} // namespace boost
 
 #endif // BOOST_SYSTEM_ERROR_CODE_HPP_INCLUDED

test/CMakeLists.txt

@@ -69,3 +69,32 @@ lib(boost_system_std_single_instance_lib2 STD_SINGLE_INSTANCE_DYN_LINK std_singl
 
 system_run(std_single_instance_test.cpp std_single_instance_1.cpp std_single_instance_2.cpp)
 boost_test(SOURCES std_single_instance_test.cpp NAME std_single_instance_test_lib LINK_LIBRARIES boost_system_std_single_instance_lib1 boost_system_std_single_instance_lib2)
+
+boost_test(TYPE run SOURCES is_error_code_enum_test.cpp)
+boost_test(TYPE run SOURCES is_error_condition_enum_test.cpp)
+boost_test(TYPE run SOURCES errc_test.cpp)
+boost_test(TYPE run SOURCES errc_test2.cpp)
+boost_test(TYPE run SOURCES error_category_test2.cpp)
+boost_test(TYPE run SOURCES error_condition_test.cpp)
+boost_test(TYPE run SOURCES error_condition_test2.cpp)
+boost_test(TYPE run SOURCES generic_category_test2.cpp)
+boost_test(TYPE run SOURCES generic_category_test3.cpp)
+boost_test(TYPE run SOURCES system_category_test2.cpp)
+boost_test(TYPE run SOURCES system_category_test3.cpp)
+
+boost_test(TYPE run SOURCES windows_error_test.cpp)
+boost_test(TYPE run SOURCES cygwin_error_test.cpp)
+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)
+boost_test(TYPE run SOURCES std_interop_test5.cpp)
+boost_test(TYPE run SOURCES std_interop_test6.cpp)
+boost_test(TYPE run SOURCES std_interop_test7.cpp)
+boost_test(TYPE run SOURCES std_interop_test8.cpp)
+boost_test(TYPE run SOURCES std_interop_test9.cpp)

test/Jamfile.v2

@@ -56,7 +56,7 @@ system-run generic_category_test.cpp ;
 system-run system_category_test.cpp ;
 system-run after_main_test.cpp ;
 system-run failed_test.cpp ;
-compile failed_constexpr_test.cpp : <toolset>gcc,<undefined-sanitizer>norecover:<build>no ;
+system-run failed_constexpr_test.cpp ;
 
 # Quick (CI) test
 run quick.cpp ;
@@ -89,3 +89,22 @@ run system_category_test3.cpp ;
 run windows_error_test.cpp ;
 run cygwin_error_test.cpp ;
 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 ;
+run std_interop_test5.cpp
+  : : :
+  # crash on xenial, but the real g++ 4.8 on both centos 7 and trusty works
+  <toolset>gcc-4.8:<build>no
+  <toolset>gcc-4.9:<build>no
+  ;
+
+run std_interop_test6.cpp ;
+run std_interop_test7.cpp ;
+run std_interop_test8.cpp ;
+run std_interop_test9.cpp ;

test/errc_test3.cpp

@@ -0,0 +1,34 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/errc.hpp>
+
+int main()
+{
+    make_error_code( boost::system::errc::success ).value();
+    make_error_code( boost::system::errc::success ).category();
+    make_error_code( boost::system::errc::success ).default_error_condition();
+    make_error_code( boost::system::errc::success ).message();
+    make_error_code( boost::system::errc::success ).failed();
+    make_error_code( boost::system::errc::success ).clear();
+
+    make_error_condition( boost::system::errc::success ).value();
+    make_error_condition( boost::system::errc::success ).category();
+    make_error_condition( boost::system::errc::success ).message();
+    make_error_condition( boost::system::errc::success ).failed();
+    make_error_condition( boost::system::errc::success ).clear();
+
+    make_error_code( boost::system::errc::no_such_file_or_directory ).value();
+    make_error_code( boost::system::errc::no_such_file_or_directory ).category();
+    make_error_code( boost::system::errc::no_such_file_or_directory ).default_error_condition();
+    make_error_code( boost::system::errc::no_such_file_or_directory ).message();
+    make_error_code( boost::system::errc::no_such_file_or_directory ).failed();
+    make_error_code( boost::system::errc::no_such_file_or_directory ).clear();
+
+    make_error_condition( boost::system::errc::no_such_file_or_directory ).value();
+    make_error_condition( boost::system::errc::no_such_file_or_directory ).category();
+    make_error_condition( boost::system::errc::no_such_file_or_directory ).message();
+    make_error_condition( boost::system::errc::no_such_file_or_directory ).failed();
+    make_error_condition( boost::system::errc::no_such_file_or_directory ).clear();
+}

test/failed_constexpr_test.cpp

@@ -35,28 +35,16 @@ BOOST_STATIC_ASSERT( !ec3 );
 
 constexpr error_condition en1( 1, system_category() );
 
-#if __GNUC__ == 7 && __cplusplus == 201703L
-// `cat_ != 0` is not a constant expression
-#else
-
 BOOST_STATIC_ASSERT( en1.failed() );
 BOOST_STATIC_ASSERT( en1 );
 BOOST_STATIC_ASSERT( !!en1 );
 
-#endif
-
 constexpr error_condition en2( 2, generic_category() );
 
-#if __GNUC__ == 7 && __cplusplus == 201703L
-// `cat_ != 0` is not a constant expression
-#else
-
 BOOST_STATIC_ASSERT( en2.failed() );
 BOOST_STATIC_ASSERT( en2 );
 BOOST_STATIC_ASSERT( !!en2 );
 
-#endif
-
 constexpr error_condition en3;
 
 BOOST_STATIC_ASSERT( !en3.failed() );

test/failed_test.cpp

@@ -120,30 +120,18 @@ template<class Ec> void test()
         ec.assign( 0, generic_category() );
         TEST_NOT_FAILED( ec );
     }
-}
 
-template<class Ec> void test2()
-{
-    Ec ec( 0, http_category() );
-    TEST_FAILED( ec );
+    {
+        Ec ec( 0, http_category() );
+        TEST_FAILED( ec );
 
-    ec.assign( 200, http_category() );
-    TEST_NOT_FAILED( ec );
+        ec.assign( 200, http_category() );
+        TEST_NOT_FAILED( ec );
 
-    ec = Ec( 404, http_category() );
-    TEST_FAILED( ec );
-}
+        ec = Ec( 404, http_category() );
+        TEST_FAILED( ec );
+    }
 
-template<class Ec> void test3()
-{
-    Ec ec( 0, http_category() );
-    BOOST_TEST( ec.failed() );
-
-    ec.assign( 200, http_category() );
-    BOOST_TEST( !ec.failed() );
-
-    ec = Ec( 404, http_category() );
-    BOOST_TEST( ec.failed() );
 }
 
 int main()
@@ -159,9 +147,7 @@ int main()
     BOOST_TEST( http_category().failed( 404 ) );
 
     test<error_code>();
-    test2<error_code>();
     test<error_condition>();
-    test3<error_condition>();
 
     {
         error_condition ec( errc::success );

test/snprintf_test.cpp

@@ -0,0 +1,60 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// http://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/detail/snprintf.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+int main()
+{
+    {
+        char buffer[ 64 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "...xy...151..." );
+    }
+
+    {
+        char buffer[ 64 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "...xy...151..." );
+    }
+
+    {
+        char buffer[ 15 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "...xy...151..." );
+    }
+
+    {
+        char buffer[ 14 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "...xy...151.." );
+    }
+
+    {
+        char buffer[ 5 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "...x" );
+    }
+
+    {
+        char buffer[ 1 ];
+        boost::system::detail::snprintf( buffer, sizeof(buffer), "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_CSTR_EQ( buffer, "" );
+    }
+
+    {
+        char buffer[ 1 ] = { 'Q' };
+        boost::system::detail::snprintf( buffer, 0, "...%s...%d...", "xy", 151 );
+
+        BOOST_TEST_EQ( buffer[0], 'Q' );
+    }
+
+    return boost::report_errors();
+}

test/std_interop_test2.cpp

@@ -0,0 +1,98 @@
+// 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() );
+#if !defined(BOOST_SYSTEM_CLANG_6)
+        f2( e1, e1.value(), e1.category() );
+#endif
+
+        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() );
+#if !defined(BOOST_SYSTEM_CLANG_6)
+        f2( e1, e1.value(), e1.category() );
+#endif
+
+        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() );
+#if !defined(BOOST_SYSTEM_CLANG_6)
+        f2( e1, e1.value(), e1.category() );
+#endif
+
+        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() );
+#if !defined(BOOST_SYSTEM_CLANG_6)
+        f2( e1, e1.value(), e1.category() );
+#endif
+
+        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() );
+#if !defined(BOOST_SYSTEM_CLANG_6)
+        f2( e1, e1.value(), e1.category() );
+#endif
+
+        BOOST_TEST_EQ( e1, e2 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test3.cpp

@@ -0,0 +1,79 @@
+// 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;
+
+        BOOST_TEST( !e2.failed() );
+        BOOST_TEST_EQ( e1.message(), e2.message() );
+
+        std::error_code e3 = e2;
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 5, std::system_category() );
+        boost::system::error_code e2 = e1;
+
+        BOOST_TEST( e2.failed() );
+        BOOST_TEST_EQ( e1.message(), e2.message() );
+
+        std::error_code e3 = e2;
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( 0, std::generic_category() );
+        boost::system::error_code e2 = e1;
+
+        BOOST_TEST( !e2.failed() );
+        BOOST_TEST_EQ( e1.message(), e2.message() );
+
+        std::error_code e3 = e2;
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1( ENOENT, std::generic_category() );
+        boost::system::error_code e2 = e1;
+
+        BOOST_TEST( e2.failed() );
+        BOOST_TEST_EQ( e1.message(), e2.message() );
+
+        std::error_code e3 = e2;
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    {
+        std::error_code e1 = make_error_code( std::errc::no_such_file_or_directory );
+        boost::system::error_code e2 = e1;
+
+        BOOST_TEST( e2.failed() );
+        BOOST_TEST_EQ( e1.message(), e2.message() );
+
+        std::error_code e3 = e2;
+        BOOST_TEST_EQ( e1, e3 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

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

test/std_interop_test5.cpp

@@ -0,0 +1,108 @@
+// 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;
+        std::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST( e1 == en );
+        BOOST_TEST_NOT( e1 != en );
+
+        boost::system::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        std::error_code e3( e2 );
+
+        BOOST_TEST( e3 == en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        std::error_code e1( 5, std::system_category() );
+        std::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST( e1 == en );
+        BOOST_TEST_NOT( e1 != en );
+
+        boost::system::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        std::error_code e3( e2 );
+
+        BOOST_TEST( e3 == en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        std::error_code e1( 0, std::generic_category() );
+        std::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST( e1 == en );
+        BOOST_TEST_NOT( e1 != en );
+
+        boost::system::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        std::error_code e3( e2 );
+
+        BOOST_TEST( e3 == en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        std::error_code e1( 5, std::generic_category() );
+        std::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST( e1 == en );
+        BOOST_TEST_NOT( e1 != en );
+
+        boost::system::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        std::error_code e3( e2 );
+
+        BOOST_TEST( e3 == en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test6.cpp

@@ -0,0 +1,129 @@
+// 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()
+{
+    {
+        boost::system::error_code e1;
+        boost::system::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST_EQ( e1, en );
+        BOOST_TEST_NOT( e1 != en );
+
+        std::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        boost::system::error_code e3( e2 );
+
+        BOOST_TEST_EQ( e3, en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        boost::system::error_code e1( 0, boost::system::system_category() );
+        boost::system::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST_EQ( e1, en );
+        BOOST_TEST_NOT( e1 != en );
+
+        std::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        boost::system::error_code e3( e2 );
+
+        BOOST_TEST_EQ( e3, en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        boost::system::error_code e1( 5, boost::system::system_category() );
+        boost::system::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST_EQ( e1, en );
+        BOOST_TEST_NOT( e1 != en );
+
+        std::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        boost::system::error_code e3( e2 );
+
+        BOOST_TEST_EQ( e3, en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        boost::system::error_code e1( 0, boost::system::generic_category() );
+        boost::system::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST_EQ( e1, en );
+        BOOST_TEST_NOT( e1 != en );
+
+        std::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        boost::system::error_code e3( e2 );
+
+        BOOST_TEST_EQ( e3, en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    {
+        boost::system::error_code e1( 5, boost::system::generic_category() );
+        boost::system::error_condition en = e1.default_error_condition();
+
+        BOOST_TEST_EQ( e1, en );
+        BOOST_TEST_NOT( e1 != en );
+
+        std::error_code e2( e1 );
+
+        BOOST_TEST_EQ( e2, e1 );
+        BOOST_TEST_NOT( e2 != e1 );
+
+        BOOST_TEST( e2 == en );
+        BOOST_TEST_NOT( e2 != en );
+
+        boost::system::error_code e3( e2 );
+
+        BOOST_TEST_EQ( e3, en );
+        BOOST_TEST_NOT( e3 != en );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test7.cpp

@@ -0,0 +1,52 @@
+// 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 <boost/config.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()
+{
+    {
+        boost::system::error_code e1 = make_error_code( boost::system::errc::bad_address );
+
+        BOOST_TEST( e1 == boost::system::errc::bad_address );
+        BOOST_TEST_NOT( e1 != boost::system::errc::bad_address );
+
+        BOOST_TEST( e1 == std::errc::bad_address );
+        BOOST_TEST_NOT( e1 != std::errc::bad_address );
+    }
+
+    {
+        boost::system::error_code e1 = make_error_code( std::errc::bad_address );
+
+        BOOST_TEST( e1 == boost::system::errc::bad_address );
+        BOOST_TEST_NOT( e1 != boost::system::errc::bad_address );
+
+#if defined(BOOST_GCC) && BOOST_GCC >= 40800 && BOOST_GCC < 50000
+
+// fails on g++ 4.8.5 and g++ 4.9.4 from ubuntu-toolchain-r-test for unknown reasons
+// works on the system g++ 4.8.4 on Trusty and the system g++ 4.8.5 on CentOS 7
+
+#else
+        BOOST_TEST( e1 == std::errc::bad_address );
+        BOOST_TEST_NOT( e1 != std::errc::bad_address );
+#endif
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test8.cpp

@@ -0,0 +1,82 @@
+// 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 <boost/config.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>
+
+enum my_errc
+{
+    my_enoent = ENOENT
+};
+
+#if defined(BOOST_GCC) && BOOST_GCC < 70000
+
+// g++ 6 and earlier do not allow specializations outside the namespace
+
+namespace boost
+{
+namespace system
+{
+
+template<> struct is_error_condition_enum<my_errc>: std::true_type {};
+
+} // namespace system
+} // namespace boost
+
+namespace std
+{
+
+template<> struct is_error_condition_enum<my_errc>: std::true_type {};
+
+} // namespace std
+
+#else
+
+template<> struct boost::system::is_error_condition_enum<my_errc>: std::true_type {};
+template<> struct std::is_error_condition_enum<my_errc>: std::true_type {};
+
+#endif
+
+boost::system::error_condition make_error_condition( my_errc e )
+{
+    return boost::system::error_condition( e, boost::system::generic_category() );
+}
+
+boost::system::error_code make_error_code( my_errc e )
+{
+    return boost::system::error_code( e, boost::system::generic_category() );
+}
+
+int main()
+{
+    {
+        boost::system::error_code e1 = make_error_code( my_enoent );
+
+        BOOST_TEST( e1 == my_enoent );
+        BOOST_TEST_NOT( e1 != my_enoent );
+    }
+
+    {
+        std::error_code e1 = make_error_code( my_enoent );
+
+        BOOST_TEST( e1 == my_enoent );
+        BOOST_TEST_NOT( e1 != my_enoent );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test9.cpp

@@ -0,0 +1,77 @@
+// 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 <boost/config.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>
+
+enum my_errc
+{
+    my_enoent = ENOENT
+};
+
+#if defined(BOOST_GCC) && BOOST_GCC < 70000
+
+// g++ 6 and earlier do not allow specializations outside the namespace
+
+namespace boost
+{
+namespace system
+{
+
+template<> struct is_error_code_enum<my_errc>: std::true_type {};
+
+} // namespace system
+} // namespace boost
+
+namespace std
+{
+
+template<> struct is_error_code_enum<my_errc>: std::true_type {};
+
+} // namespace std
+
+#else
+
+template<> struct boost::system::is_error_code_enum<my_errc>: std::true_type {};
+template<> struct std::is_error_code_enum<my_errc>: std::true_type {};
+
+#endif
+
+boost::system::error_code make_error_code( my_errc e )
+{
+    return boost::system::error_code( e, boost::system::generic_category() );
+}
+
+int main()
+{
+    {
+        boost::system::error_code e1 = my_enoent;
+
+        BOOST_TEST( e1 == my_enoent );
+        BOOST_TEST_NOT( e1 != my_enoent );
+    }
+
+    {
+        std::error_code e1 = my_enoent;
+
+        BOOST_TEST( e1 == my_enoent );
+        BOOST_TEST_NOT( e1 != my_enoent );
+    }
+
+    return boost::report_errors();
+}
+
+#endif