Update documentation

doc/system/changes.adoc

@@ -12,6 +12,8 @@ https://www.boost.org/LICENSE_1_0.txt
 
 * {cpp}03 is no longer supported; a {cpp}11 compiler is required. (This includes GCC 4.8 or later, and MSVC 14.0 (VS 2015) or later.)
 * The deprecated header `boost/system/cygwin_error.hpp` has been removed.
+* The original and obsolete (32 bit) MinGW is no longer supported. MinGW-w64 (both 64 and 32 bit) is still supported.
+* `operator&` now works for `result<void>` (by way of taking a nullary function.)
 
 ## Changes in Boost 1.84
 

doc/system/reference.adoc

@@ -2655,6 +2655,24 @@ result<double> convert( double amount, currency_type from, currency_type to )
 }
 ```
 
+```
+template<class E, class F, class U = ...>
+  result<U, E> operator&( result<void, E> const& r, F&& f );
+```
+[none]
+* {blank}
++
+Returns the error in `r`, or if `r` contains a value, replaces it with the result of calling `f`.
++
+Let `U` be the type of `f()`.
++
+Effects: ::
+  - If `r.has_value()` is `true`, returns `f()`.
+  - Otherwise, returns `r.error()`.
+Remarks: ::
+  Only enabled when `U` is not an instance of `result`.
+
+
 ```
 template<class T, class E, class F, class R = ...> R operator&( result<T, E> const& r, F&& f );
 template<class T, class E, class F, class R = ...> R operator&( result<T, E>&& r, F&& f );
@@ -2703,6 +2721,24 @@ int get_port( JsonValue const& config, int def )
 }
 ```
 
+```
+template<class E, class F, class R = ...> R operator&( result<void, E> const& r, F&& f );
+```
+[none]
+* {blank}
++
+Returns the error in `r`, or if `r` contains a value, another `result` obtained
+by invoking the function `f`.
++
+Let `R` be the type of `f()`.
++
+Effects: ::
+  - If `r.has_value()` is `true`, returns `f()`.
+  - Otherwise, returns `r.error()`.
+Remarks: ::
+  Only enabled when `R` is an instance of `result` and `E` is convertible to `R::error_type`.
+
+
 #### operator&=
 
 ```