Return v_.index() == 1 from has_error() to avoid -Wmaybe-uninitialized in error()

.github/workflows/ci.yml

@@ -110,7 +110,7 @@ jobs:
             install: clang-14
           - toolset: clang
             cxxstd: "03,11,14,17,2a"
-            os: macos-10.15
+            os: macos-11
 
     runs-on: ${{matrix.os}}
 
@@ -220,7 +220,7 @@ jobs:
           - os: ubuntu-18.04
           - os: ubuntu-20.04
           - os: ubuntu-22.04
-          - os: macos-10.15
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 
@@ -267,7 +267,7 @@ jobs:
           - os: ubuntu-18.04
           - os: ubuntu-20.04
           - os: ubuntu-22.04
-          - os: macos-10.15
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 
@@ -324,7 +324,7 @@ jobs:
           - os: ubuntu-18.04
           - os: ubuntu-20.04
           - os: ubuntu-22.04
-          - os: macos-10.15
+          - os: macos-11
 
     runs-on: ${{matrix.os}}
 

README.md

@@ -0,0 +1,25 @@
+# Boost.System
+
+The Boost.System library, part of [Boost C++ Libraries](https://boost.org),
+implements an extensible framework for error reporting in the form of an
+`error_code` class and supporting facilities.
+
+It has been proposed for the C++11 standard, has been accepted, and
+is now available as part of the standard library in the `<system_error>`
+header. However, the Boost implementation has continued to evolve and
+gain enhancements and additional functionality, such as support for
+attaching [source locations](https://www.boost.org/doc/libs/release/libs/assert/doc/html/assert.html#source_location_support)
+to `error_code`, and a `result<T>` class that can carry either a value
+or an error code.
+
+See [the documentation of System](http://boost.org/libs/system) for more
+information.
+
+Since `<system_error>` is a relatively undocumented portion of the C++
+standard library, the documentation of Boost.System may be useful to you
+even if you use the standard components.
+
+## License
+
+Distributed under the
+[Boost Software License, Version 1.0](http://boost.org/LICENSE_1_0.txt).

doc/system/changes.adoc

@@ -16,6 +16,7 @@ https://www.boost.org/LICENSE_1_0.txt
   to avoid the one-time allocation that shows up on leak checkers.
 * Added a constructor that allows replacing the source location of an
   `error_code`, and a corresponding `assign`.
+* Added a converting constructor to `result`.
 
 ## Changes in Boost 1.79
 

doc/system/reference.adoc

@@ -1608,6 +1608,12 @@ public:
     template<class... A>
       constexpr result( in_place_error_t, A&&... a );
 
+    template<class T2, class E2>
+      constexpr result( result<T2, E2> const& r2 );
+
+    template<class T2, class E2>
+      constexpr result( result<T2, E2>&& r2 );
+
     // queries
 
     constexpr bool has_value() const noexcept;
@@ -1716,6 +1722,30 @@ Ensures: ::
 Remarks: ::
   This constructor is only enabled when `std::is_constructible<E, A...>::value` is `true`.
 
+```
+template<class T2, class E2>
+  constexpr result( result<T2, E2> const& r2 );
+```
+[none]
+* {blank}
++
+Ensures: ::
+  If `r2.has_value()` is `true`, `*this` holds the value `T( *r2 )`, otherwise `*this` holds the value `E( r2.error() )`.
+Remarks: ::
+  This constructor is only enabled when `std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value` is `true`.
+
+```
+template<class T2, class E2>
+  constexpr result( result<T2, E2>&& r2 );
+```
+[none]
+* {blank}
++
+Ensures: ::
+  If `r2.has_value()` is `true`, `*this` holds the value `T( std::move( *r2 ) )`, otherwise `*this` holds the value `E( r2.error() )`.
+Remarks: ::
+  This constructor is only enabled when `std::is_convertible<T2, T>::value && std::is_convertible<E2, E>::value` is `true`.
+
 #### Queries
 
 ```

include/boost/system/result.hpp

@@ -135,7 +135,8 @@ public:
     template<class... A, class En = typename std::enable_if<
         std::is_constructible<T, A...>::value &&
         !(detail::is_errc_t<A...>::value && std::is_arithmetic<T>::value) &&
-        !std::is_constructible<E, A...>::value
+        !std::is_constructible<E, A...>::value &&
+        sizeof...(A) >= 1
         >::type>
     explicit constexpr result( A&&... a )
         noexcept( std::is_nothrow_constructible<T, A...>::value )
@@ -146,7 +147,8 @@ public:
     // 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
+        std::is_constructible<E, A...>::value &&
+        sizeof...(A) >= 1
         >::type>
     explicit constexpr result( A&&... a )
         noexcept( std::is_nothrow_constructible<E, A...>::value )
@@ -220,7 +222,7 @@ public:
 
     constexpr bool has_error() const noexcept
     {
-        return v_.index() != 0;
+        return v_.index() == 1;
     }
 
     constexpr explicit operator bool() const noexcept
@@ -509,7 +511,7 @@ public:
 
     constexpr bool has_error() const noexcept
     {
-        return v_.index() != 0;
+        return v_.index() == 1;
     }
 
     constexpr explicit operator bool() const noexcept

test/result_default_construct.cpp

@@ -12,6 +12,11 @@ struct X
 {
 };
 
+struct Y
+{
+    Y( int );
+};
+
 int main()
 {
     {
@@ -37,5 +42,17 @@ int main()
         BOOST_TEST( !r.has_error() );
     }
 
+    {
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<int>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<int, int>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<X>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<X, int>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<void>>));
+        BOOST_TEST_TRAIT_TRUE((std::is_default_constructible<result<void, int>>));
+
+        BOOST_TEST_TRAIT_FALSE((std::is_default_constructible<result<Y>>));
+        BOOST_TEST_TRAIT_FALSE((std::is_default_constructible<result<Y, int>>));
+    }
+
     return boost::report_errors();
 }