Protect against dangling references in op* and value()

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();
+}