Protect against dangling references in op* and value()

include/boost/system/result.hpp

@@ -210,12 +210,31 @@ 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 +293,31 @@ 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);
     }

test/CMakeLists.txt

@@ -125,3 +125,4 @@ 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)

test/Jamfile.v2

@@ -149,3 +149,4 @@ 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) ;

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();
+}