Enable implicit construction for aggregates using {{ ... }}

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)... )
     {

test/CMakeLists.txt

@@ -126,3 +126,5 @@ 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

@@ -150,3 +150,5 @@ 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/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_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();
+}