omit unnecessary bits when explicit operator conversions are supported

test/compile-fail/implicit_int_1.cpp

@@ -0,0 +1,16 @@
+// 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);
+
+  return i; // expect to see: error: no viable conversion from returned value of type 'boost::logic::tribool' to function return type 'int'
+}