Move from boost-sandbox

[SVN r18959]

include/boost/iterator/detail/enable_if.hpp

@@ -0,0 +1,88 @@
+// (C) Copyright David Abrahams 2002.
+// (C) Copyright Jeremy Siek    2002.
+// (C) Copyright Thomas Witt    2002.
+// Permission to copy, use, modify,
+// sell and distribute this software is granted provided this
+// copyright notice appears in all copies. This software is provided
+// "as is" without express or implied warranty, and with no claim as
+// to its suitability for any purpose.
+#ifndef BOOST_ENABLE_IF_23022003THW_HPP
+#define BOOST_ENABLE_IF_23022003THW_HPP
+
+#include <boost/detail/workaround.hpp>
+#include <boost/mpl/identity.hpp>
+
+#include <boost/iterator/detail/config_def.hpp>
+
+//
+// Boost iterators uses its own enable_if cause we need
+// special semantics for deficient compilers.
+// 23/02/03 thw
+//
+
+namespace boost
+{
+
+  namespace detail
+  {
+    //
+    // Base machinery for all kinds of enable if
+    //
+    template<bool>
+    struct enabled
+    {
+      template<typename T>
+      struct base
+      {
+        typedef T type;
+      };
+    };
+    
+    //
+    // For compilers that don't support "Substitution Failure Is Not An Error"
+    // enable_if falls back to always enabled. See comments
+    // on operator implementation for consequences.
+    //
+    template<>
+    struct enabled<false>
+    {
+      template<typename T>
+      struct base
+      {
+#ifdef BOOST_NO_SFINAE
+
+        typedef T type;
+
+        // This way to do it would give a nice error message containing
+        // invalid overload, but has the big disadvantage that
+        // there is no reference to user code in the error message.
+        //
+        // struct invalid_overload;
+        // typedef invalid_overload type;
+        //
+#endif
+      };
+    };
+
+
+    template <class Cond,
+              class Return>
+    struct enable_if
+# if !defined(BOOST_NO_SFINAE) && !defined(BOOST_NO_IS_CONVERTIBLE)
+      : enabled<(Cond::value)>::template base<Return>
+# else
+      : mpl::identity<Return>
+# endif 
+    {
+# if BOOST_WORKAROUND(BOOST_MSVC, <= 1200)
+        typedef Return type;
+# endif 
+    };
+
+  } // namespace detail
+
+} // namespace boost
+
+#include <boost/iterator/detail/config_undef.hpp>
+
+#endif // BOOST_ENABLE_IF_23022003THW_HPP