Add usage examples

This attribute will check format string bugs and is required for clean compile if -Wformat-nonliteral is enabled.

.github/workflows/ci.yml

@@ -152,6 +152,10 @@ jobs:
             cxxstd: "14,17,latest"
             addrmd: 32,64
             os: windows-2019
+          - toolset: msvc-14.3
+            cxxstd: "14,17,latest"
+            addrmd: 32,64
+            os: windows-2022
           - toolset: gcc
             cxxstd: "03,11,14,17,2a"
             addrmd: 64

doc/system.adoc

@@ -13,10 +13,13 @@ Beman Dawes, Christopher Kohlhoff, Peter Dimov
 :toclevels: 4
 :idprefix:
 :docinfo: private-footer
+:source-highlighter: rouge
+:source-language: c++
 
 :leveloffset: +1
 
 include::system/introduction.adoc[]
+include::system/usage.adoc[]
 include::system/changes.adoc[]
 include::system/rationale.adoc[]
 include::system/reference.adoc[]

doc/system/changes.adoc

@@ -11,7 +11,7 @@ https://www.boost.org/LICENSE_1_0.txt
 ## Changes in Boost 1.78
 
 * Added support for source locations to `error_code`.
-* Added `error_code::to_string`, `error_condition::to_string`.
+* Added `error_code::to_string`, `error_condition::to_string`, `error_code::what`.
 * `system_error::what()` now contains the source location, if present.
 * Added `result<T, E = error_code>`, a class holding either a value or an
   error, defined in `<boost/system/result.hpp>`.

doc/system/reference.adoc

@@ -571,9 +571,13 @@ public:
     error_code( int val, const error_category & cat,
       boost::source_location const * loc ) noexcept;
 
-    template <class ErrorCodeEnum>
+    template<class ErrorCodeEnum>
       constexpr error_code( ErrorCodeEnum e ) noexcept;
 
+    template<class ErrorCodeEnum>
+      error_code( ErrorCodeEnum e, boost::source_location const * loc )
+        noexcept;
+
     error_code( std::error_code const& ec ) noexcept;
 
     // modifiers
@@ -583,9 +587,13 @@ public:
     void assign( int val, const error_category & cat,
       boost::source_location const * loc ) noexcept;
 
-    template<typename ErrorCodeEnum>
+    template<class ErrorCodeEnum>
       constexpr error_code & operator=( ErrorCodeEnum e ) noexcept;
 
+    template<class ErrorCodeEnum>
+      void assign( ErrorCodeEnum e,
+        boost::source_location const * loc ) noexcept;
+
     constexpr void clear() noexcept;
 
     // observers
@@ -657,9 +665,13 @@ public:
 
     // stream insertion
 
-    template <class charT, class traits>
+    template<class charT, class traits>
       friend std::basic_ostream<charT, traits>&
         operator<<( basic_ostream<charT, traits>& os, const error_code & ec );
+
+    // what
+
+    std::string what() const;
 };
 
 // non-member functions
@@ -699,7 +711,7 @@ Requires: :: `loc` points to a valid `boost::source_location` object with static
 Ensures: :: `value() == val`; `category() == cat`; `has_location()`; `&location() == loc`.
 
 ```
-template <class ErrorCodeEnum>
+template<class ErrorCodeEnum>
   constexpr error_code( ErrorCodeEnum e ) noexcept;
 ```
 [none]
@@ -708,6 +720,19 @@ template <class ErrorCodeEnum>
 Ensures: :: `*this == make_error_code( e )`.
 Remarks: :: This constructor is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.
 
+```
+template<class ErrorCodeEnum>
+  error_code( ErrorCodeEnum e, boost::source_location const * loc ) noexcept;
+```
+[none]
+* {blank}
++
+Requires: :: `loc` points to a valid `boost::source_location` object with static storage duration.
+Ensures: :: `*this == make_error_code( e )`.
+Remarks: :: This constructor is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.
+  When `make_error_code( e )` is not a default-constructed `error_code` and doesn't wrap a
+  `std::error_code`, `has_location()` is `true` and `&location()` is `loc`.
+
 ```
 error_code( std::error_code const & ec ) noexcept;
 ```
@@ -737,7 +762,7 @@ void assign( int val, const error_category & cat,
 Effects: :: `*this = error_code( val, cat, loc )`.
 
 ```
-template<typename ErrorCodeEnum>
+template<class ErrorCodeEnum>
   constexpr error_code & operator=( ErrorCodeEnum e ) noexcept;
 ```
 [none]
@@ -746,6 +771,18 @@ template<typename ErrorCodeEnum>
 Ensures: :: `*this == make_error_code( e )`.
 Remarks: :: This operator is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.
 
+```
+template<class ErrorCodeEnum>
+  void assign( ErrorCodeEnum e,
+    boost::source_location const * loc ) noexcept;
+```
+[none]
+* {blank}
++
+Requires: :: `loc` points to a valid `boost::source_location` object with static storage duration.
+Effects: :: `*this = error_code( e, loc )`.
+Remarks: :: This function is only enabled when `is_error_code_enum<ErrorCodeEnum>::value` is `true`.
+
 ```
 constexpr void clear() noexcept;
 ```
@@ -1021,6 +1058,18 @@ template <class charT, class traits>
 Effects: :: `os << to_string()`.
 Returns: :: `os`.
 
+#### what
+
+```
+std::string what() const;
+```
+[none]
+* {blank}
++
+Returns: :: A string representation of `*this`, suitable for logging and
+  diagnostic output. Typically incorporates `message()`, `to_string()`, and
+  `location().to_string()` (if available.)
+
 #### Nonmembers
 
 ```

doc/system/usage.adoc

@@ -0,0 +1,612 @@
+////
+Copyright 2021 Peter Dimov
+Distributed under the Boost Software License, Version 1.0.
+https://www.boost.org/LICENSE_1_0.txt
+////
+
+[#usage]
+# Usage Examples
+:idprefix: usage_
+
+All of the following examples assume that these lines
+```
+#include <boost/system.hpp>
+namespace sys = boost::system;
+```
+are in effect.
+
+## Returning Errors from OS APIs under POSIX
+
+Let's suppose that we're implementing a portable `file` wrapper
+over the OS file APIs. Its general outline is shown below:
+
+```
+class file
+{
+private:
+
+    int fd_;
+
+public:
+
+    // ...
+
+    std::size_t read( void * buffer, std::size_t size, sys::error_code& ec );
+    std::size_t write( void const * buffer, std::size_t size, sys::error_code& ec );
+};
+```
+
+Since we're implementing the POSIX version of `file`, its
+data member is a POSIX file descriptor `int fd_;`, although other
+implementations will differ.
+
+Our `read` and `write` functions return the number of bytes transferred, and signal
+errors via the output parameter `ec`, of type `boost::system::error_code`.
+
+An implementation of `file::read` might look like this:
+
+```
+std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
+{
+    ssize_t r = ::read( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        ec.assign( errno, sys::system_category() );
+        return 0;
+    }
+
+    ec = {}; // ec.clear(); under C++03
+    return r;
+}
+```
+
+We first call the POSIX API `read`; if it returns an error, we store the `errno`
+value in `ec`, using the system category, and return 0 as bytes transferred.
+Otherwise, we clear `ec` to signal success, and return the result of `::read`.
+
+NOTE: Clearing `ec` on successful returns is an important step; do not omit it.
+
+Under POSIX, the system category corresponds to POSIX `errno` values, which is
+why we use it.
+
+In principle, since the generic category _also_ corresponds to `errno` values
+under all platforms, we could have used it here; however, by convention under
+POSIX, if the `errno` value comes from the OS (the "system"), we use the system
+category for it. That's because the system category values may be a
+platform-specific superset of the generic (platform-independent) values.
+
+The implementation of `file::write` is basically the same. We show it here for
+completeness:
+
+```
+std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
+{
+    ssize_t r = ::write( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        ec.assign( errno, sys::system_category() );
+        return 0;
+    }
+
+    ec = {}; // ec.clear(); under C++03
+    return r;
+}
+```
+
+## Returning Errors from OS APIs under Windows
+
+Under Windows, our `file` object will store a `HANDLE` instead of an `int`:
+
+```
+class file
+{
+private:
+
+    HANDLE fh_;
+
+public:
+
+    // as before
+};
+```
+
+and the implementation of `file::read` will look like this:
+
+```
+std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
+{
+    DWORD r = 0;
+
+    if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
+    {
+        // success
+        ec = {}; // ec.clear(); under C++03
+    }
+    else
+    {
+        // failure
+        ec.assign( ::GetLastError(), sys::system_category() );
+    }
+
+    // In both cases, r is bytes transferred
+    return r;
+}
+```
+
+Here, the system category corresponds to the values defined in the system
+header `<winerror.h>` and returned by `GetLastError()`. Since we use the
+Win32 API `ReadFile` to implement `file::read`, and it returns the error
+code via `GetLastError()`, we again store that value in `ec` as belonging
+to the system category.
+
+The implementation of `file::write` is, again, the same.
+
+```
+std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
+{
+    DWORD r = 0;
+
+    if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
+    {
+        ec = {}; // ec.clear(); under C++03
+    }
+    else
+    {
+        ec.assign( ::GetLastError(), sys::system_category() );
+    }
+
+    return r;
+}
+```
+
+## Returning Specific Errors under POSIX
+
+Our implementation of `file::read` has a problem; it accepts `std::size_t`
+values for `size`, but the behavior of `::read` is unspecified when the
+requested value does not fit in `ssize_t`. To avoid reliance on unspecified
+behavior, let's add a check for this condition and return an error:
+
+```
+std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
+{
+    if( size > SSIZE_MAX )
+    {
+        ec.assign( EINVAL, sys::generic_category() );
+        return 0;
+    }
+
+    ssize_t r = ::read( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        ec.assign( errno, sys::system_category() );
+        return 0;
+    }
+
+    ec = {}; // ec.clear(); under C++03
+    return r;
+}
+```
+
+In this case, since we're returning the fixed `errno` value `EINVAL`, which
+is part of the portable subset defined by the generic category, we mark the
+error value in `ec` as belonging to the generic category.
+
+It's possible to use system as well, as `EINVAL` is also a system category
+value under POSIX; however, using the generic category for values belonging
+to the portable `errno` subset is slightly preferrable.
+
+Our implementation of `file::write` needs to uindergo a similar treatment.
+There, however, we'll apply another change. When there's no space left on
+the disk, `::write` returns a number of bytes written that is lower than
+what we requested with `size`, but our function signals no error. We'll make
+it return `ENOSPC` in this case.
+
+```
+std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
+{
+    if( size > SSIZE_MAX )
+    {
+        ec.assign( EINVAL, sys::generic_category() );
+        return 0;
+    }
+
+    ssize_t r = ::write( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        ec.assign( errno, sys::system_category() );
+        return 0;
+    }
+
+    if( r < size )
+    {
+        ec.assign( ENOSPC, sys::system_category() );
+    }
+    else
+    {
+        ec = {}; // ec.clear(); under C++03
+    }
+
+    return r;
+}
+```
+
+We've used the system category to make it appear that the `ENOSPC` value
+has come from the `::write` API, mostly to illustrate that this is also a
+possible approach. Using a generic value would have worked just as well.
+
+## Returning Specific Errors under Windows
+
+Not much to say; the situation under Windows is exactly the same. The only
+difference is that we _must_ use the generic category for returning `errno`
+values. The system category does not work; the integer values in the system
+category are entirely different from those in the generic category.
+
+```
+std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
+{
+    DWORD r = 0;
+
+    if( size > MAXDWORD )
+    {
+        ec.assign( EINVAL, sys::generic_category() );
+    }
+    else if( ::ReadFile( fh_, buffer, size, &r, 0 ) )
+    {
+        ec = {}; // ec.clear(); under C++03
+    }
+    else
+    {
+        ec.assign( ::GetLastError(), sys::system_category() );
+    }
+
+    return r;
+}
+
+std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
+{
+    DWORD r = 0;
+
+    if( size > MAXDWORD )
+    {
+        ec.assign( EINVAL, sys::generic_category() );
+    }
+    else if( ::WriteFile( fh_, buffer, size, &r, 0 ) )
+    {
+        if( r < size )
+        {
+            ec.assign( ENOSPC, sys::generic_category() );
+        }
+        else
+        {
+            ec = {}; // ec.clear(); under C++03
+        }
+    }
+    else
+    {
+        ec.assign( ::GetLastError(), sys::system_category() );
+    }
+
+    return r;
+}
+```
+
+## Attaching a Source Location to Error Codes
+
+Unlike the standard `<system_error>`, Boost.System allows source locations
+(file:line:function) to be stored in `error_code`, so that functions handling
+the error can display or log the source code location where the error occurred.
+To take advantage of this functionality, our POSIX `file::read` function needs
+to be augmented as follows:
+
+```
+std::size_t file::read( void * buffer, std::size_t size, sys::error_code& ec )
+{
+    if( size > SSIZE_MAX )
+    {
+        static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
+        ec.assign( EINVAL, sys::generic_category(), &loc );
+        return 0;
+    }
+
+    ssize_t r = ::read( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        static constexpr boost::source_location loc = BOOST_CURRENT_LOCATION;
+        ec.assign( errno, sys::system_category(), &loc );
+        return 0;
+    }
+
+    ec = {}; // ec.clear(); under C++03
+    return r;
+}
+```
+
+That is, before every `ec.assign` statement, we need to declare a
+`static constexpr` variable holding the current source location, then pass
+a pointer to it to `assign`. Since `error_code` is small and there's no space
+in it for more than a pointer, we can't just store the `source_location` in it
+by value.
+
+`BOOST_CURRENT_LOCATION` is a macro expanding to the current source location
+(a combination of `++__FILE__++`, `++__LINE__++`, and `BOOST_CURRENT_FUNCTION`.)
+It's defined and documented in link:../../../assert/index.html[Boost.Assert].
+
+Under {cpp}03, instead of `static constexpr`, one needs to use `static const`.
+Another option is `BOOST_STATIC_CONSTEXPR`, a
+link:../../../config/index.html[Boost.Config] macro that expands to either
+`static constexpr` or `static const`, as appropriate.
+
+To avoid repeating this boilerplate each time we do `ec.assign`, we can define
+a macro:
+
+```
+#define ASSIGN(ec, ...) { \
+    BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION; \
+    (ec).assign(__VA_ARGS__, &loc); }
+```
+
+which we can now use to augment, for example, the POSIX implementation of `file::write`:
+
+```
+std::size_t file::write( void const * buffer, std::size_t size, sys::error_code& ec )
+{
+    if( size > SSIZE_MAX )
+    {
+        ASSIGN( ec, EINVAL, sys::generic_category() );
+        return 0;
+    }
+
+    ssize_t r = ::write( fd_, buffer, size );
+
+    if( r < 0 )
+    {
+        ASSIGN( ec, errno, sys::system_category() );
+        return 0;
+    }
+
+    if( r < size )
+    {
+        ASSIGN( ec, ENOSPC, sys::generic_category() );
+    }
+    else
+    {
+        ec = {}; // ec.clear(); under C++03
+    }
+
+    return r;
+}
+```
+
+## Obtaining Textual Representations of Error Codes for Logging and Display
+
+Assuming that we have an `error_code` instance `ec`, returned to us by some
+function, we have a variety of means to obtain textual representations of the
+error code represented therein.
+
+`ec.to_string()` gives us the result of streaming `ec` into a `std::ostream`,
+e.g. if `std::cout << ec << std::endl;` outputs `system:6`, this is what
+`ec.to_string()` will return. (`system:6` under Windows is `ERROR_INVALID_HANDLE`
+from `<winerror.h>`.)
+
+To obtain a human-readable error message corresponding to this code, we can
+use `ec.message()`. For `ERROR_INVALID_HANDLE`, it would give us "The handle is
+invalid" - possibly localized.
+
+If `ec` contains a source location, we can obtain its textual representation
+via `ec.location().to_string()`. This will give us something like
+
+```text
+C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)'
+```
+
+if there is a location in `ec`, and
+
+```text
+(unknown source location)
+```
+
+if there isn't. (`ec.has_location()` is `true` when `ec` contains a location.)
+
+Finally, `ec.what()` will give us a string that contains all of the above,
+something like
+
+```text
+The handle is invalid [system:6 at C:\Projects\testbed2019\testbed2019.cpp:98 in function 'unsigned __int64 __cdecl file::read(void *,unsigned __int64,class boost::system::error_code &)']
+```
+
+Most logging and diagnostic output that is not intended for the end user would
+probably end up using `what()`. (`ec.what()`, augmented with the prefix
+supplied at construction, is also what `boost::system::system_error::what()`
+would return.)
+
+## Composing Functions Returning Error Codes
+
+Let's suppose that we need to implement a file copy function, with the following
+interface:
+
+```
+std::size_t file_copy( file& src, file& dest, sys::error_code& ec );
+```
+
+`file_copy` uses `src.read` to read bytes from `src`, then writes these bytes
+to `dest` using `dest.write`. This continues until one of these operations signals
+an error, or until end of file is reached. It returns the number of bytes written,
+and uses `ec` to signal an error.
+
+Here is one possible implementation:
+
+```
+std::size_t file_copy( file& src, file& dest, sys::error_code& ec )
+{
+    std::size_t r = 0;
+
+    for( ;; )
+    {
+        unsigned char buffer[ 1024 ];
+
+        std::size_t n = src.read( buffer, sizeof( buffer ), ec );
+
+        // read failed, leave the error in ec and return
+        if( ec.failed() ) return r;
+
+        // end of file has been reached, exit loop
+        if( n == 0 ) return r;
+
+        r += dest.write( buffer, n, ec );
+
+        // write failed, leave the error in ec and return
+        if( ec.failed() ) return r;
+    }
+}
+```
+
+Note that there is no longer any difference between POSIX and Windows
+implementations; their differences are contained in `file::read` and
+`file::write`. `file_copy` is portable and works under any platform.
+
+The general pattern in writing such higher-level functions is that
+they pass the output `error_code` parameter `ec` they received from
+the caller directly as the output parameter to the lower-level functions
+they are built upon. This way, when they detect a failure in an intermediate
+operation (by testing `ec.failed()`), they can immediately return to the
+caller, because the error code is already in its proper place.
+
+Note that `file_copy` doesn't even need to clear `ec` on success, by
+using `ec = {};`. Since we've already tested `ec.failed()`, we know that
+`ec` contains a value that means success.
+
+## Providing Dual (Throwing and Nonthrowing) Overloads
+
+Functions that signal errors via an output `error_code& ec` parameter
+require that the caller check `ec` after calling them, and take appropriate
+action (such as return immediately, as above.) Forgetting to check `ec`
+results in logic errors.
+
+While this is a preferred coding style for some, others prefer exceptions,
+which one cannot forget to check.
+
+An approach that has been introduced by
+link:../../../filesystem/index.html[Boost.Filesystem] (which later turned
+into `std::filesystem`) is to provide both alternatives: a nonthrowing
+function taking `error_code& ec`, as `file_copy` above, and a throwing
+function that does not take an `error_code` output parameter, and throws
+exceptions on failure.
+
+This is how this second throwing function is typically implemented:
+
+```
+std::size_t file_copy( file& src, file& dest )
+{
+    sys::error_code ec;
+    std::size_t r = file_copy( src, dest, ec );
+
+    if( ec.failed() ) throw sys::system_error( ec, __func__ );
+
+    return r;
+}
+```
+
+That is, we simply call the nonthrowing overload of `file_copy`, and if
+it signals failure in `ec`, throw a `system_error` exception.
+
+We use our function name `++__func__++` (`file_copy`) as the prefix, although
+that's a matter of taste.
+
+Note that typically under this style the overloads taking `error_code& ec`
+are decorated with `noexcept`, so that it's clear that they don't throw
+exceptions (although we haven't done so in the preceding examples in order
+to keep the code {cpp}03-friendly.)
+
+## result<T> as an Alternative to Dual APIs
+
+Instead of providing two functions for every operation, an alternative
+approach is to make the function return `sys::result<T>` instead of `T`.
+`result<T>` is a class holding either `T` or `error_code`, similar to
+link:../../../variant2/index.html[`variant<T, error_code>`].
+
+Clients that prefer to check for errors and not rely on exceptions can
+test whether a `result<T> r` contains a value via `if( r )` or its more
+verbose equivalent `if( r.has_value() )`, then obtain the value via
+`*r` or `r.value()`. If `r` doesn't contain a value, the `error_code`
+it holds can be obtained with `r.error()`.
+
+Those who prefer exceptions just call `r.value()` directly, without
+checking. In the no-value case, this will automatically throw a
+`system_error` corresponding to the `error_code` in `r`.
+
+Assuming our base `file` API is unchanged, this variation of `file_copy`
+would look like this:
+
+```
+sys::result<std::size_t> file_copy( file& src, file& dest )
+{
+    std::size_t r = 0;
+    sys::error_code ec;
+
+    for( ;; )
+    {
+        unsigned char buffer[ 1024 ];
+
+        std::size_t n = src.read( buffer, sizeof( buffer ), ec );
+
+        if( ec.failed() ) return ec;
+        if( n == 0 ) return r;
+
+        r += dest.write( buffer, n, ec );
+
+        if( ec.failed() ) return ec;
+    }
+}
+```
+
+The only difference here is that we return `ec` on error, instead of
+`r`.
+
+Note, however, that we can no longer return both an error code and a
+number of transferred bytes; that is, we can no longer signal _partial
+success_. This is often not an issue at higher levels, but lower-level
+primitives such as `file::read` and `file::write` might be better off
+written using the old style.
+
+Nevertheless, to demonstrate how `result` returning APIs are composed,
+we'll show how `file_copy` would look if `file::read` and `file::write`
+returned `result<size_t>`:
+
+```
+class file
+{
+public:
+
+    // ...
+
+    sys::result<std::size_t> read( void * buffer, std::size_t size );
+    sys::result<std::size_t> write( void const * buffer, std::size_t size );
+};
+
+sys::result<std::size_t> file_copy( file& src, file& dest )
+{
+    std::size_t m = 0;
+
+    for( ;; )
+    {
+        unsigned char buffer[ 1024 ];
+
+        auto r = src.read( buffer, sizeof( buffer ) );
+        if( !r ) return r;
+
+        std::size_t n = *r;
+        if( n == 0 ) return m;
+
+        auto r2 = dest.write( buffer, n );
+        if( !r2 ) return r2;
+
+        std::size_t n2 = *r2;
+        m += n2;
+    }
+}
+```

include/boost/system/detail/error_category_impl.hpp

@@ -20,7 +20,6 @@
 
 namespace boost
 {
-
 namespace system
 {
 
@@ -91,7 +90,6 @@ inline char const * error_category::message( int ev, char * buffer, std::size_t
 }
 
 } // namespace system
-
 } // namespace boost
 
 // interoperability with std::error_code, std::error_condition
@@ -100,28 +98,54 @@ inline char const * error_category::message( int ev, char * buffer, std::size_t
 
 #include <boost/system/detail/std_category.hpp>
 
-inline boost::system::error_category::operator std::error_category const & () const
+namespace boost
 {
-    if( id_ == boost::system::detail::system_category_id )
+namespace system
+{
+
+inline error_category::operator std::error_category const & () const
+{
+    if( id_ == detail::generic_category_id )
     {
-        static const boost::system::detail::std_category system_instance( this, 0x1F4D7 );
-        return system_instance;
+// This condition must be the same as the one in error_condition.hpp
+#if defined(BOOST_GCC) && BOOST_GCC < 50000
+
+    static const boost::system::detail::std_category generic_instance( this, 0x1F4D3 );
+    return generic_instance;
+
+#else
+
+    return std::generic_category();
+
+#endif
     }
 
-    if( id_ == boost::system::detail::generic_category_id )
+    if( id_ == detail::system_category_id )
     {
-        static const boost::system::detail::std_category generic_instance( this, 0x1F4D3 );
-        return generic_instance;
+// This condition must be the same as the one in error_code.hpp
+#if defined(__CYGWIN__) || defined(__MINGW32__) || (defined(_MSC_VER) && _MSC_VER == 1800) || (defined(BOOST_GCC) && BOOST_GCC < 50000)
+
+    // Under Cygwin (and MinGW!), std::system_category() is POSIX
+    // Under VS2013, std::system_category() isn't quite right
+
+    static const boost::system::detail::std_category system_instance( this, 0x1F4D7 );
+    return system_instance;
+
+#else
+
+    return std::system_category();
+
+#endif
     }
 
-    boost::system::detail::std_category* p = ps_.load( std::memory_order_acquire );
+    detail::std_category* p = ps_.load( std::memory_order_acquire );
 
     if( p != 0 )
     {
         return *p;
     }
 
-    boost::system::detail::std_category* q = new detail::std_category( this, 0 );
+    detail::std_category* q = new detail::std_category( this, 0 );
 
     if( ps_.compare_exchange_strong( p, q, std::memory_order_release, std::memory_order_acquire ) )
     {
@@ -134,6 +158,9 @@ inline boost::system::error_category::operator std::error_category const & () co
     }
 }
 
+} // namespace system
+} // namespace boost
+
 #endif // #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
 #endif // #ifndef BOOST_SYSTEM_DETAIL_ERROR_CATEGORY_IMPL_HPP_INCLUDED

include/boost/system/detail/error_code.hpp

@@ -107,10 +107,30 @@ public:
     }
 
     template<class ErrorCodeEnum> BOOST_SYSTEM_CONSTEXPR error_code( ErrorCodeEnum e,
+        typename detail::enable_if<
+            is_error_code_enum<ErrorCodeEnum>::value
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+            || std::is_error_code_enum<ErrorCodeEnum>::value
+#endif
+        >::type* = 0 ) BOOST_NOEXCEPT: d1_(), lc_flags_( 0 )
+    {
+        *this = make_error_code( e );
+    }
+
+    template<class ErrorCodeEnum> error_code( ErrorCodeEnum e, source_location const * loc,
         typename detail::enable_if<is_error_code_enum<ErrorCodeEnum>::value>::type* = 0 ) BOOST_NOEXCEPT:
         d1_(), lc_flags_( 0 )
     {
-        *this = make_error_code( e );
+        error_code e2 = make_error_code( e );
+
+        if( e2.lc_flags_ == 0 || e2.lc_flags_ == 1 )
+        {
+            *this = e2;
+        }
+        else
+        {
+            *this = error_code( e2.d1_.val_, *e2.d1_.cat_, loc );
+        }
     }
 
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
@@ -143,6 +163,13 @@ public:
         return *this;
     }
 
+    template<typename ErrorCodeEnum>
+        typename detail::enable_if<is_error_code_enum<ErrorCodeEnum>::value, void>::type
+        assign( ErrorCodeEnum val, source_location const * loc ) BOOST_NOEXCEPT
+    {
+        *this = error_code( val, loc );
+    }
+
     BOOST_SYSTEM_CONSTEXPR void clear() BOOST_NOEXCEPT
     {
         *this = error_code();
@@ -504,8 +531,16 @@ public:
         }
         else if( lc_flags_ == 0 )
         {
-            //return std::error_code();
+// This condition must be the same as the one in error_category_impl.hpp
+#if defined(__CYGWIN__) || defined(__MINGW32__) || (defined(_MSC_VER) && _MSC_VER == 1800) || (defined(BOOST_GCC) && BOOST_GCC < 50000)
+
             return std::error_code( 0, boost::system::system_category() );
+
+#else
+
+            return std::error_code();
+
+#endif
         }
         else
         {
@@ -571,6 +606,23 @@ public:
     {
         return os << ec.to_string();
     }
+
+    std::string what() const
+    {
+        std::string r = message();
+
+        r += " [";
+        r += to_string();
+
+        if( has_location() )
+        {
+            r += " at ";
+            r += location().to_string();
+        }
+
+        r += "]";
+        return r;
+    }
 };
 
 inline std::size_t hash_value( error_code const & ec )

include/boost/system/detail/error_condition.hpp

@@ -222,7 +222,23 @@ public:
 
     operator std::error_condition () const
     {
+// This condition must be the same as the one in error_category_impl.hpp
+#if defined(BOOST_GCC) && BOOST_GCC < 50000
+
         return std::error_condition( value(), category() );
+
+#else
+
+        if( cat_ )
+        {
+            return std::error_condition( val_, *cat_ );
+        }
+        else
+        {
+            return std::error_condition( val_, std::generic_category() );
+        }
+
+#endif
     }
 
     inline friend bool operator==( std::error_code const & lhs, error_condition const & rhs ) BOOST_NOEXCEPT

include/boost/system/detail/snprintf.hpp

@@ -49,6 +49,9 @@ inline void snprintf( char * buffer, std::size_t len, char const * format, ... )
 
 #else
 
+#if defined(__GNUC__) && __GNUC__ >= 3
+__attribute__((__format__ (__printf__, 3, 4)))
+#endif
 inline void snprintf( char * buffer, std::size_t len, char const * format, ... )
 {
     va_list args;

include/boost/system/result.hpp

@@ -62,35 +62,10 @@ public:
     {
     }
 
-    // explicit, value
-    template<class A, class En = typename std::enable_if<
-        std::is_constructible<T, A>::value &&
-        !std::is_convertible<A, T>::value &&
-        !std::is_constructible<E, A>::value
-        >::type>
-    explicit constexpr result( A&& a )
-        noexcept( std::is_nothrow_constructible<T, A>::value )
-        : v_( in_place_value, std::forward<A>(a) )
-    {
-    }
-
-    // explicit, error
-    template<class A, class En2 = void, class En = typename std::enable_if<
-        std::is_constructible<E, A>::value &&
-        !std::is_convertible<A, E>::value &&
-        !std::is_constructible<T, A>::value
-        >::type>
-    explicit constexpr result( A&& a )
-        noexcept( std::is_nothrow_constructible<E, A>::value )
-        : v_( in_place_error, std::forward<A>(a) )
-    {
-    }
-
     // implicit, value
-    template<class A, class En2 = void, class En3 = void, class En = typename std::enable_if<
+    template<class A = T, typename std::enable_if<
         std::is_convertible<A, T>::value &&
-        !std::is_constructible<E, A>::value
-        >::type>
+        !std::is_constructible<E, A>::value, int>::type = 0>
     constexpr result( A&& a )
         noexcept( std::is_nothrow_constructible<T, A>::value )
         : v_( in_place_value, std::forward<A>(a) )
@@ -98,35 +73,32 @@ public:
     }
 
     // implicit, error
-    template<class A, class En2 = void, class En3 = void, class En4 = void, class En = typename std::enable_if<
+    template<class A = E, class = void, typename std::enable_if<
         std::is_convertible<A, E>::value &&
-        !std::is_constructible<T, A>::value
-        >::type>
+        !std::is_constructible<T, A>::value, int>::type = 0>
     constexpr result( A&& a )
         noexcept( std::is_nothrow_constructible<E, A>::value )
         : v_( in_place_error, std::forward<A>(a) )
     {
     }
 
-    // more than one arg, value
+    // explicit, value
     template<class... A, class En = typename std::enable_if<
         std::is_constructible<T, A...>::value &&
-        !std::is_constructible<E, A...>::value &&
-        sizeof...(A) >= 2
+        !std::is_constructible<E, A...>::value
         >::type>
-    constexpr result( A&&... a )
+    explicit constexpr result( A&&... a )
         noexcept( std::is_nothrow_constructible<T, A...>::value )
         : v_( in_place_value, std::forward<A>(a)... )
     {
     }
 
-    // more than one arg, error
+    // explicit, error
     template<class... A, class En2 = void, class En = typename std::enable_if<
         !std::is_constructible<T, A...>::value &&
-        std::is_constructible<E, A...>::value &&
-        sizeof...(A) >= 2
+        std::is_constructible<E, A...>::value
         >::type>
-    constexpr result( A&&... a )
+    explicit constexpr result( A&&... a )
         noexcept( std::is_nothrow_constructible<E, A...>::value )
         : v_( in_place_error, std::forward<A>(a)... )
     {
@@ -210,12 +182,31 @@ public:
         }
     }
 
-    BOOST_CXX14_CONSTEXPR T&& value() &&
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<std::is_move_constructible<U>::value, T>::type
+        value() &&
     {
         return std::move( value() );
     }
 
-    BOOST_CXX14_CONSTEXPR T const&& value() const&&
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<!std::is_move_constructible<U>::value, T&&>::type
+        value() &&
+    {
+        return std::move( value() );
+    }
+
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<std::is_move_constructible<U>::value, T>::type
+        value() const && = delete;
+
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<!std::is_move_constructible<U>::value, T const&&>::type
+        value() const &&
     {
         return std::move( value() );
     }
@@ -274,12 +265,31 @@ public:
         return *p;
     }
 
-    BOOST_CXX14_CONSTEXPR T&& operator*() && noexcept
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<std::is_move_constructible<U>::value, T>::type
+        operator*() && noexcept(std::is_nothrow_move_constructible<T>::value)
     {
         return std::move(**this);
     }
 
-    BOOST_CXX14_CONSTEXPR T const&& operator*() const && noexcept
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<!std::is_move_constructible<U>::value, T&&>::type
+        operator*() && noexcept
+    {
+        return std::move(**this);
+    }
+
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<std::is_move_constructible<U>::value, T>::type
+        operator*() const && noexcept = delete;
+
+    template<class U = T>
+        BOOST_CXX14_CONSTEXPR
+        typename std::enable_if<!std::is_move_constructible<U>::value, T const&&>::type
+        operator*() const && noexcept
     {
         return std::move(**this);
     }

include/boost/system/system_error.hpp

@@ -6,9 +6,8 @@
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
+#include <boost/system/errc.hpp>
 #include <boost/system/detail/error_code.hpp>
-#include <boost/system/detail/error_category_impl.hpp>
-#include <boost/system/detail/snprintf.hpp>
 #include <string>
 #include <stdexcept>
 #include <cassert>
@@ -26,14 +25,6 @@ private:
 
 private:
 
-    static std::string to_string( int v )
-    {
-        char buffer[ 32 ];
-        detail::snprintf( buffer, sizeof( buffer ), "%d", v );
-
-        return buffer;
-    }
-
     static std::string build_message( char const * prefix, error_code const & ec )
     {
         std::string r;
@@ -44,32 +35,7 @@ private:
             r += ": ";
         }
 
-        r += ec.message();
-
-        if( ec.has_location() )
-        {
-            r += " [";
-            r += ec.to_string();
-            r += " at ";
-
-            boost::source_location loc = ec.location();
-
-            r += loc.file_name();
-
-            r += ':';
-            r += to_string( loc.line() );
-
-            if( loc.column() != 0 )
-            {
-                r += ':';
-                r += to_string( loc.column() );
-            }
-
-            r += " in function '";
-            r += loc.function_name();
-            r += "\']";
-        }
-
+        r += ec.what();
         return r;
     }
 

test/CMakeLists.txt

@@ -103,6 +103,14 @@ boost_test(TYPE run SOURCES ec_location_test.cpp)
 
 boost_test(TYPE run SOURCES error_condition_test3.cpp)
 boost_test(TYPE run SOURCES error_code_test2.cpp)
+boost_test(TYPE run SOURCES system_error_test2.cpp)
+boost_test(TYPE run SOURCES std_interop_test10.cpp)
+
+boost_test(TYPE run SOURCES ec_location_test2.cpp)
+boost_test(TYPE run SOURCES ec_what_test.cpp)
+boost_test(TYPE run SOURCES system_error_test3.cpp)
+
+boost_test(TYPE run SOURCES std_interop_test11.cpp)
 
 # result
 
@@ -119,3 +127,6 @@ boost_test(TYPE run SOURCES result_value_access.cpp)
 boost_test(TYPE run SOURCES result_error_access.cpp)
 boost_test(TYPE run SOURCES result_swap.cpp)
 boost_test(TYPE run SOURCES result_eq.cpp)
+boost_test(TYPE run SOURCES result_range_for.cpp)
+boost_test(TYPE run SOURCES result_value_construct2.cpp)
+boost_test(TYPE run SOURCES result_error_construct2.cpp)

test/Jamfile.v2

@@ -73,8 +73,8 @@ system-run failed_constexpr_test.cpp ;
 # Quick (CI) test
 run quick.cpp ;
 
-compile warnings_test.cpp
-  : <warnings>pedantic
+run warnings_test.cpp
+  : : : <warnings>pedantic
     <toolset>msvc:<warnings-as-errors>on
     <toolset>gcc:<warnings-as-errors>on
     <toolset>clang:<warnings-as-errors>on ;
@@ -125,6 +125,14 @@ run ec_location_test.cpp ;
 
 run error_condition_test3.cpp ;
 run error_code_test2.cpp ;
+run system_error_test2.cpp ;
+run std_interop_test10.cpp ;
+
+run ec_location_test2.cpp ;
+run ec_what_test.cpp ;
+run system_error_test3.cpp ;
+
+run std_interop_test11.cpp ;
 
 # result
 
@@ -143,3 +151,6 @@ run result_value_access.cpp : : : $(CPP11) ;
 run result_error_access.cpp : : : $(CPP11) ;
 run result_swap.cpp : : : $(CPP11) <toolset>gcc-10:<cxxflags>"-Wno-maybe-uninitialized" ;
 run result_eq.cpp : : : $(CPP11) ;
+run result_range_for.cpp : : : $(CPP11) ;
+run result_value_construct2.cpp : : : $(CPP11) ;
+run result_error_construct2.cpp : : : $(CPP11) ;

test/cmake_install_test/main.cpp

@@ -40,5 +40,5 @@ int main()
     boost::system::system_error x( bc, "prefix" );
 
     BOOST_TEST_EQ( x.code(), bc );
-    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.message() );
+    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.what() );
 }

test/cmake_subdir_test/CMakeLists.txt

@@ -12,6 +12,7 @@ add_subdirectory(../../../config boostorg/config)
 add_subdirectory(../../../core boostorg/core)
 add_subdirectory(../../../mp11 boostorg/mp11)
 add_subdirectory(../../../predef boostorg/predef)
+add_subdirectory(../../../static_assert boostorg/static_assert)
 add_subdirectory(../../../throw_exception boostorg/throw_exception)
 add_subdirectory(../../../variant2 boostorg/variant2)
 add_subdirectory(../../../winapi boostorg/winapi)

test/ec_location_test2.cpp

@@ -0,0 +1,287 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <cerrno>
+
+namespace sys = boost::system;
+
+enum E
+{
+    none = 0,
+    einval = EINVAL
+};
+
+namespace boost
+{
+namespace system
+{
+
+template<> struct is_error_code_enum< ::E >
+{
+    static const bool value = true;
+};
+
+} // namespace system
+} // namespace boost
+
+sys::error_code make_error_code( E e )
+{
+    return e == 0? sys::error_code(): sys::error_code( e, sys::generic_category() );
+}
+
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+enum E2
+{
+    e2inval = EINVAL
+};
+
+namespace boost
+{
+namespace system
+{
+
+template<> struct is_error_code_enum< ::E2 >
+{
+    static const bool value = true;
+};
+
+} // namespace system
+} // namespace boost
+
+std::error_code make_error_code( E2 e )
+{
+    return std::error_code( e, std::generic_category() );
+}
+
+#endif
+
+int main()
+{
+    {
+        sys::error_code ec( einval );
+
+        BOOST_TEST_EQ( ec.value(), EINVAL );
+        BOOST_TEST_EQ( &ec.category(), &sys::generic_category() );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( einval, &loc );
+
+        BOOST_TEST_EQ( ec.value(), EINVAL );
+        BOOST_TEST_EQ( &ec.category(), &sys::generic_category() );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 77 );
+    }
+
+    {
+        sys::error_code ec( none );
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( none, &loc );
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec = sys::error_code( einval, &loc );
+
+        BOOST_TEST_EQ( ec.value(), EINVAL );
+        BOOST_TEST_EQ( &ec.category(), &sys::generic_category() );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 127 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec = sys::error_code( none, &loc );
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec.assign( einval, &loc );
+
+        BOOST_TEST_EQ( ec.value(), EINVAL );
+        BOOST_TEST_EQ( &ec.category(), &sys::generic_category() );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 175 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec.assign( none, &loc );
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+    {
+        sys::error_code ec( e2inval );
+
+        BOOST_TEST_EQ( ec, std::error_code( EINVAL, std::generic_category() ) );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( e2inval, &loc );
+
+        BOOST_TEST_EQ( ec, std::error_code( EINVAL, std::generic_category() ) );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec = sys::error_code( e2inval, &loc );
+
+        BOOST_TEST_EQ( ec, std::error_code( EINVAL, std::generic_category() ) );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST_EQ( &ec.category(), &sys::system_category() );
+
+        BOOST_TEST( !ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        ec.assign( e2inval, &loc );
+
+        BOOST_TEST_EQ( ec, std::error_code( EINVAL, std::generic_category() ) );
+
+        BOOST_TEST( ec.failed() );
+
+        BOOST_TEST( !ec.has_location() );
+        BOOST_TEST_EQ( ec.location().line(), 0 );
+    }
+
+#endif
+
+    return boost::report_errors();
+}

test/ec_what_test.cpp

@@ -0,0 +1,91 @@
+// Copyright 2020, 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/generic_category.hpp>
+#include <boost/system/system_category.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+namespace sys = boost::system;
+
+int main()
+{
+    {
+        sys::error_code ec;
+
+        BOOST_TEST_EQ( ec.value(), 0 );
+        BOOST_TEST( ec.category() == sys::system_category() );
+
+        BOOST_TEST_EQ( ec.what(), ec.message() + " [system:0]" );
+    }
+
+    {
+        sys::error_code ec( 5, sys::generic_category() );
+
+        BOOST_TEST_EQ( ec.value(), 5 );
+        BOOST_TEST( ec.category() == sys::generic_category() );
+
+        BOOST_TEST_EQ( ec.what(), ec.message() + " [generic:5]" );
+    }
+
+    {
+        sys::error_code ec( 5, sys::system_category() );
+
+        BOOST_TEST_EQ( ec.value(), 5 );
+        BOOST_TEST( ec.category() == sys::system_category() );
+
+        BOOST_TEST_EQ( ec.what(), ec.message() + " [system:5]" );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( 5, sys::generic_category(), &loc );
+
+        BOOST_TEST_EQ( ec.value(), 5 );
+        BOOST_TEST( ec.category() == sys::generic_category() );
+        BOOST_TEST_EQ( &ec.location(), &loc );
+
+        BOOST_TEST_EQ( ec.what(), ec.message() + " [generic:5 at " + loc.to_string() + "]" );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( 5, sys::system_category(), &loc );
+
+        BOOST_TEST_EQ( ec.value(), 5 );
+        BOOST_TEST( ec.category() == sys::system_category() );
+        BOOST_TEST_EQ( &ec.location(), &loc );
+
+        BOOST_TEST_EQ( ec.what(), ec.message() + " [system:5 at " + loc.to_string() + "]" );
+    }
+
+#if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+    {
+        std::error_code ec;
+        sys::error_code ec2( ec );
+
+        BOOST_TEST_EQ( ec2.what(), ec2.message() + " [std:system:0]" );
+    }
+
+    {
+        std::error_code ec( 5, std::generic_category() );
+        sys::error_code ec2( ec );
+
+        BOOST_TEST_EQ( ec2.what(), ec2.message() + " [std:generic:5]" );
+    }
+
+    {
+        std::error_code ec( 5, std::system_category() );
+        sys::error_code ec2( ec );
+
+        BOOST_TEST_EQ( ec2.what(), ec2.message() + " [std:system:5]" );
+    }
+
+#endif
+
+    return boost::report_errors();
+}

test/quick.cpp

@@ -37,7 +37,7 @@ int main()
     boost::system::system_error x( bc, "prefix" );
 
     BOOST_TEST_EQ( x.code(), bc );
-    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.message() );
+    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.what() );
 
     return boost::report_errors();
 }

test/result_error_construct2.cpp

@@ -0,0 +1,57 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <string>
+
+using namespace boost::system;
+
+struct X
+{
+    int a, b;
+};
+
+result<X> fx1()
+{
+    return {{ EINVAL, generic_category() }};
+}
+
+struct Y
+{
+    std::string v;
+};
+
+struct E
+{
+    int v;
+};
+
+result<Y, E> fy1()
+{
+    return {{ 42 }};
+}
+
+int main()
+{
+    {
+        result<X> r = fx1();
+
+        BOOST_TEST( !r.has_value() );
+        BOOST_TEST( r.has_error() );
+
+        BOOST_TEST_EQ( r.error(), std::error_code( EINVAL, generic_category() ) );
+    }
+
+    {
+        result<Y, E> r = fy1();
+
+        BOOST_TEST( !r.has_value() );
+        BOOST_TEST( r.has_error() );
+
+        BOOST_TEST_EQ( r.error().v, 42 );
+    }
+
+    return boost::report_errors();
+}

test/result_range_for.cpp

@@ -0,0 +1,30 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <vector>
+
+using namespace boost::system;
+
+result<std::vector<int>> f()
+{
+    return std::vector<int>{ 1, 2, 3 };
+}
+
+int main()
+{
+    {
+        int s = 0;
+
+        for( int x: f().value() )
+        {
+            s += x;
+        }
+
+        BOOST_TEST_EQ( s, 6 );
+    }
+
+    return boost::report_errors();
+}

test/result_value_construct2.cpp

@@ -0,0 +1,152 @@
+// Copyright 2021 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/result.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <vector>
+#include <string>
+
+using namespace boost::system;
+
+struct X
+{
+    int a;
+};
+
+struct Y
+{
+    int a, b;
+};
+
+struct E
+{
+    std::string v;
+};
+
+result<X> fx0()
+{
+    return {};
+}
+
+result<X> fx1()
+{
+    return {{ 1 }};
+}
+
+result<Y> fy0()
+{
+    return {};
+}
+
+result<Y> fy2()
+{
+    return {{ 1, 2 }};
+}
+
+result<X, E> fxe0()
+{
+    return {};
+}
+
+result<X, E> fxe1()
+{
+    return {{ 1 }};
+}
+
+result<std::vector<int>> fv0()
+{
+    return {};
+}
+
+result<std::vector<int>> fv1()
+{
+    return {{ 1 }};
+}
+
+result<std::vector<int>> fv2()
+{
+    return {{ 1, 2 }};
+}
+
+int main()
+{
+    {
+        result<X> r = fx0();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 0 );
+    }
+
+    {
+        result<X> r = fx1();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 1 );
+    }
+
+    {
+        result<Y> r = fy0();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 0 );
+        BOOST_TEST_EQ( r->b, 0 );
+    }
+
+    {
+        result<Y> r = fy2();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 1 );
+        BOOST_TEST_EQ( r->b, 2 );
+    }
+
+    {
+        result<X, E> r = fxe0();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 0 );
+    }
+
+    {
+        result<X, E> r = fxe1();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->a, 1 );
+    }
+
+    {
+        result<std::vector<int>> r = fv0();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->size(), 0 );
+    }
+
+    {
+        result<std::vector<int>> r = fv1();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->size(), 1 );
+        BOOST_TEST_EQ( r->at(0), 1 );
+    }
+
+    {
+        result<std::vector<int>> r = fv2();
+
+        BOOST_TEST( r.has_value() );
+
+        BOOST_TEST_EQ( r->size(), 2 );
+        BOOST_TEST_EQ( r->at(0), 1 );
+        BOOST_TEST_EQ( r->at(1), 2 );
+    }
+
+    return boost::report_errors();
+}

test/std_interop_test.cpp

@@ -44,7 +44,8 @@ static void test_generic_category()
 
     int ev = ENOENT;
 
-    BOOST_TEST_EQ( bt.message( ev ), st.message( ev ) );
+    // Under MSVC, it's "no such file or directory" instead of "No such file or directory"
+    BOOST_TEST_EQ( bt.message( ev ).substr( 1 ), st.message( ev ).substr( 1 ) );
 
     {
         boost::system::error_code bc( ev, bt );
@@ -82,34 +83,13 @@ static void test_system_category()
 
     BOOST_TEST_CSTR_EQ( bt.name(), st.name() );
 
+    for( int ev = 1; ev < 6; ++ev )
     {
-        int ev = 5;
-        BOOST_TEST_EQ( bt.message( ev ), st.message( ev ) );
+        std::string bm = bt.message( ev );
+        std::string sm = st.message( ev );
 
-        {
-            boost::system::error_code bc( ev, bt );
-
-            BOOST_TEST_EQ( bc.value(), ev );
-            BOOST_TEST_EQ( &bc.category(), &bt );
-
-            std::error_code sc( bc );
-
-            BOOST_TEST_EQ( sc.value(), ev );
-            BOOST_TEST_EQ( &sc.category(), &st );
-        }
-
-        {
-            boost::system::error_condition bn = bt.default_error_condition( ev );
-            BOOST_TEST( bt.equivalent( ev, bn ) );
-
-            std::error_condition sn( bn );
-            BOOST_TEST( st.equivalent( ev, sn ) );
-        }
-    }
-
-    {
-        int ev = 4;
-        BOOST_TEST_EQ( bt.message( ev ), st.message( ev ) );
+        // We strip whitespace and the trailing dot, MSVC not so much
+        BOOST_TEST_EQ( bm, sm.substr( 0, bm.size() ) );
 
         {
             boost::system::error_code bc( ev, bt );

test/std_interop_test10.cpp

@@ -0,0 +1,66 @@
+// 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_code e2( 0, boost::system::system_category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+
+        std::error_code e3( e1 );
+        std::error_code e4( e2 );
+
+        BOOST_TEST_EQ( e3, e4 );
+    }
+
+    {
+        boost::system::error_code e1;
+
+        std::error_code e2( e1 );
+        std::error_code e3( e1.value(), e1.category() );
+
+        BOOST_TEST_EQ( e2, e3 );
+    }
+
+    {
+        boost::system::error_condition e1;
+        boost::system::error_condition e2( 0, boost::system::generic_category() );
+
+        BOOST_TEST_EQ( e1, e2 );
+
+        std::error_condition e3( e1 );
+        std::error_condition e4( e2 );
+
+        BOOST_TEST( e3 == e4 );
+    }
+
+    {
+        boost::system::error_condition e1;
+
+        std::error_condition e2( e1 );
+        std::error_condition e3( e1.value(), e1.category() );
+
+        BOOST_TEST( e2 == e3 );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test11.cpp

@@ -0,0 +1,47 @@
+// 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 <ios>
+
+#if !defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+BOOST_PRAGMA_MESSAGE( "Skipping test, BOOST_SYSTEM_HAS_SYSTEM_ERROR not defined" )
+int main() {}
+
+#elif defined(BOOST_LIBSTDCXX_VERSION) && BOOST_LIBSTDCXX_VERSION < 50000
+
+BOOST_PRAGMA_MESSAGE( "Skipping test, BOOST_LIBSTDCXX_VERSION < 50000" )
+int main() {}
+
+#else
+
+#include <system_error>
+
+int main()
+{
+    {
+        boost::system::error_code ec = std::io_errc::stream;
+
+        BOOST_TEST( ec == std::io_errc::stream );
+        BOOST_TEST_NOT( ec != std::io_errc::stream );
+
+        ec.clear();
+
+        BOOST_TEST_NOT( ec == std::io_errc::stream );
+        BOOST_TEST( ec != std::io_errc::stream );
+
+        ec = std::io_errc::stream;
+
+        BOOST_TEST( ec == std::io_errc::stream );
+        BOOST_TEST_NOT( ec != std::io_errc::stream );
+    }
+
+    return boost::report_errors();
+}
+
+#endif

test/std_interop_test6.cpp

@@ -30,13 +30,20 @@ int main()
         BOOST_TEST_EQ( e2, e1 );
         BOOST_TEST_NOT( e2 != e1 );
 
-        BOOST_TEST( e2 == en );
+#if defined(_LIBCPP_VERSION)
+
+        // Under MS STL and libstdc++, std::error_code() does not compare
+        // equal to std::error_condition(). Go figure.
+
+        BOOST_TEST_EQ( e2, en );
         BOOST_TEST_NOT( e2 != en );
 
         boost::system::error_code e3( e2 );
 
         BOOST_TEST_EQ( e3, en );
         BOOST_TEST_NOT( e3 != en );
+
+#endif
     }
 
     {
@@ -51,13 +58,17 @@ int main()
         BOOST_TEST_EQ( e2, e1 );
         BOOST_TEST_NOT( e2 != e1 );
 
-        BOOST_TEST( e2 == en );
+#if defined(_LIBCPP_VERSION)
+
+        BOOST_TEST_EQ( e2, en );
         BOOST_TEST_NOT( e2 != en );
 
         boost::system::error_code e3( e2 );
 
         BOOST_TEST_EQ( e3, en );
         BOOST_TEST_NOT( e3 != en );
+
+#endif
     }
 
     {
@@ -72,7 +83,7 @@ int main()
         BOOST_TEST_EQ( e2, e1 );
         BOOST_TEST_NOT( e2 != e1 );
 
-        BOOST_TEST( e2 == en );
+        BOOST_TEST_EQ( e2, en );
         BOOST_TEST_NOT( e2 != en );
 
         boost::system::error_code e3( e2 );
@@ -93,7 +104,7 @@ int main()
         BOOST_TEST_EQ( e2, e1 );
         BOOST_TEST_NOT( e2 != e1 );
 
-        BOOST_TEST( e2 == en );
+        BOOST_TEST_EQ( e2, en );
         BOOST_TEST_NOT( e2 != en );
 
         boost::system::error_code e3( e2 );
@@ -114,7 +125,7 @@ int main()
         BOOST_TEST_EQ( e2, e1 );
         BOOST_TEST_NOT( e2 != e1 );
 
-        BOOST_TEST( e2 == en );
+        BOOST_TEST_EQ( e2, en );
         BOOST_TEST_NOT( e2 != en );
 
         boost::system::error_code e3( e2 );

test/system_error_test.cpp

@@ -81,27 +81,25 @@ int main( int, char *[] )
   system_error c6_0( 0, system_category(), "c6_0" ); 
   system_error c6_1( 1, system_category(), "c6_1" );
 
-  TEST( c1_0, 0, "The operation completed successfully" );
-  TEST( c1_1, 1, "Incorrect function" );
-  TEST( c1_2u, 2, "The system cannot find the file specified" );
+  TEST( c1_0, 0, "The operation completed successfully [system:0]" );
+  TEST( c1_1, 1, "Incorrect function [system:1]" );
+  TEST( c1_2u, 2, "The system cannot find the file specified [system:2]" );
 
-  TEST( c2_0, 0, "c2_0: The operation completed successfully" );
-  TEST( c2_1, 1, "c2_1: Incorrect function" );
+  TEST( c2_0, 0, "c2_0: The operation completed successfully [system:0]" );
+  TEST( c2_1, 1, "c2_1: Incorrect function [system:1]" );
 
-  TEST( c3_0, 0, "c3_0: The operation completed successfully" );
-  TEST( c3_1, 1, "c3_1: Incorrect function" );
+  TEST( c3_0, 0, "c3_0: The operation completed successfully [system:0]" );
+  TEST( c3_1, 1, "c3_1: Incorrect function [system:1]" );
 
-  TEST( c4_0, 0, "The operation completed successfully" );
-  TEST( c4_1, 1, "Incorrect function" );
-  TEST( c4_2u, 2, "The system cannot find the file specified" );
+  TEST( c4_0, 0, "The operation completed successfully [system:0]" );
+  TEST( c4_1, 1, "Incorrect function [system:1]" );
+  TEST( c4_2u, 2, "The system cannot find the file specified [system:2]" );
 
-  TEST( c5_0, 0, "c5_0: The operation completed successfully" );
-  TEST( c5_1, 1, "c5_1: Incorrect function" );
+  TEST( c5_0, 0, "c5_0: The operation completed successfully [system:0]" );
+  TEST( c5_1, 1, "c5_1: Incorrect function [system:1]" );
 
-  TEST( c6_0, 0, "c6_0: The operation completed successfully" );
-  TEST( c6_1, 1, "c6_1: Incorrect function" );
+  TEST( c6_0, 0, "c6_0: The operation completed successfully [system:0]" );
+  TEST( c6_1, 1, "c6_1: Incorrect function [system:1]" );
 
   return ::boost::report_errors();
 }
-
-

test/system_error_test2.cpp

@@ -0,0 +1,41 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/system_error.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <cerrno>
+
+namespace sys = boost::system;
+
+int main()
+{
+    {
+        sys::error_code ec( 5, sys::generic_category() );
+        sys::system_error x1( ec );
+        (void)x1;
+    }
+
+    {
+        sys::error_code ec( 5, sys::system_category() );
+        sys::system_error x1( ec );
+        (void)x1;
+    }
+
+    {
+        sys::system_error x1( make_error_code( sys::errc::invalid_argument ) );
+        (void)x1;
+    }
+
+    {
+        sys::system_error x1( 5, sys::generic_category() );
+        (void)x1;
+    }
+
+    {
+        sys::system_error x1( 5, sys::system_category() );
+        (void)x1;
+    }
+
+    return boost::report_errors();
+}

test/system_error_test3.cpp

@@ -0,0 +1,42 @@
+// Copyright 2021 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/system_error.hpp>
+#include <boost/core/lightweight_test.hpp>
+#include <cerrno>
+
+namespace sys = boost::system;
+
+int main()
+{
+    {
+        sys::error_code ec( 5, sys::generic_category() );
+        sys::system_error x1( ec );
+
+		BOOST_TEST_EQ( std::string( x1.what() ), ec.what() );
+    }
+
+    {
+        BOOST_STATIC_CONSTEXPR boost::source_location loc = BOOST_CURRENT_LOCATION;
+
+        sys::error_code ec( 5, sys::system_category(), &loc );
+        sys::system_error x1( ec, "prefix" );
+
+        BOOST_TEST_EQ( std::string( x1.what() ), "prefix: " + ec.what() );
+    }
+
+    {
+        sys::system_error x1( 5, sys::generic_category() );
+
+        BOOST_TEST_EQ( std::string( x1.what() ), sys::error_code( 5, sys::generic_category() ).what() );
+    }
+
+    {
+        sys::system_error x1( 5, sys::system_category(), "prefix" );
+
+        BOOST_TEST_EQ( std::string( x1.what() ), "prefix: " + sys::error_code( 5, sys::system_category() ).what() );
+    }
+
+    return boost::report_errors();
+}

test/warnings_test.cpp

@@ -46,7 +46,7 @@ int main()
     boost::system::system_error x( bc, "prefix" );
 
     BOOST_TEST_EQ( x.code(), bc );
-    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.message() );
+    BOOST_TEST_EQ( std::string( x.what() ), "prefix: " + bc.what() );
 
     return boost::report_errors();
 }