Use BOOST_EXPLICIT_OPERATOR_BOOL for optional

I often have the problem that when I change a std::wstring to boost::optional<std::wstring> and the variable is used as a parameter with Boost.Format, the result silently changes from the string contents to "1".

This change prevents implicit conversion to bool if the compiler supports explicit conversion operators.

test/Jamfile.v2

@@ -29,5 +29,6 @@ import testing ;
     [ compile-fail optional_test_ref_fail4.cpp ]
     [ compile-fail optional_test_inplace_fail.cpp ]
     [ compile-fail optional_test_inplace_fail2.cpp ]
+    [ compile-fail optional_test_fail_implicit_bool_convert.cpp ]
   ;
 }

test/optional_test.cpp

@@ -1086,8 +1086,8 @@ namespace optional_swap_test
   //
   void swap(boost::optional<class_whose_explicit_ctor_should_be_used> & x, boost::optional<class_whose_explicit_ctor_should_be_used> & y)
   {
-    bool hasX = x;
-    bool hasY = y;
+    bool hasX(x);
+    bool hasY(y);
 
     if ( !hasX && !hasY )
      return;

test/optional_test_fail_implicit_bool_convert.cpp

@@ -0,0 +1,28 @@
+// Copyright (C) 2014, Andrzej Krzemienski.
+//
+// Use, modification, and distribution is subject to 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)
+//
+// See http://www.boost.org/lib/optional for documentation.
+//
+// You are welcome to contact the author at:
+//  akrzemi1@gmail.com
+//
+#include "boost/optional.hpp"
+
+//
+// THIS TEST SHOULD FAIL TO COMPILE
+//
+
+#if !defined(BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS)
+
+bool test_implicit_conversion_to_bool()
+{
+  boost::optional<int> opt;
+  return opt;
+}
+
+#else
+#  error "Test skipped: this compiler does not support explicit conversion operators."
+#endif