Use explicit operator bool when available

include/boost/system/error_code.hpp

@@ -2,6 +2,7 @@
 
 //  Copyright Beman Dawes 2006, 2007
 //  Copyright Christoper Kohlhoff 2007
+//  Copyright Peter Dimov 2017, 2018
 
 //  Distributed under the Boost Software License, Version 1.0. (See accompanying
 //  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
@@ -387,6 +388,15 @@ namespace boost
       const error_category &  category() const BOOST_SYSTEM_NOEXCEPT { return *m_cat; }
       std::string             message() const  { return m_cat->message(value()); }
 
+#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+      explicit operator bool() const BOOST_SYSTEM_NOEXCEPT  // true if error
+      {
+        return m_val != 0;
+      }
+
+#else
+
       typedef void (*unspecified_bool_type)();
       static void unspecified_bool_true() {}
 
@@ -400,6 +410,8 @@ namespace boost
         return m_val == 0;
       }
 
+#endif
+
       // relationals:
       //  the more symmetrical non-member syntax allows enum
       //  conversions work for both rhs and lhs.
@@ -486,6 +498,15 @@ namespace boost
         { return m_cat->default_error_condition(value()); }
       std::string             message() const  { return m_cat->message(value()); }
 
+#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+      explicit operator bool() const BOOST_SYSTEM_NOEXCEPT  // true if error
+      {
+        return m_val != 0;
+      }
+
+#else
+
       typedef void (*unspecified_bool_type)();
       static void unspecified_bool_true() {}
 
@@ -499,6 +520,8 @@ namespace boost
         return m_val == 0;
       }
 
+#endif
+
       // relationals:
       inline friend bool operator==( const error_code & lhs,
                                      const error_code & rhs ) BOOST_SYSTEM_NOEXCEPT