omit unnecessary bits when explicit operator conversions are supported

test/compile-fail/implicit.cpp

@@ -0,0 +1,23 @@
+// Copyright 2018 James E. King III. 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)
+
+#include <boost/logic/tribool.hpp>
+
+int main(int, char*[])
+{
+  using boost::logic::indeterminate;
+  using boost::logic::tribool;
+
+  tribool i(indeterminate);
+
+#if !defined( BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS )
+  bool b = i; // expect to see: no viable conversion from 'boost::logic::tribool' to 'bool'
+  return (int)b;
+#else
+#error in c++03 explicit conversions are allowed
+#endif
+  // NOTREACHED
+  return 0;
+}