[boost] [foreach] Can should_copy_impl return type be simplified?

I'm studying the implementation of foreach and I had a question about should_copy_impl. For reference, here's the overload that's vexing me: // If the collection is an array or is noncopyable, it must be an lvalue. // If the collection is a lightweight proxy, treat it as an rvalue // BUGBUG what about a noncopyable proxy? template inline BOOST_DEDUCED_TYPENAME boost::enable_if, IsProxy>::type * should_copy_impl(LValue *, IsProxy *, bool *) { return 0; } Why is enable_if needed? Can't the return type just be "IsProxy *"?