[boost] [parameter] Possible breaking change

As part of the new [un]named template parameter support, we are considering a breaking change to the binding<ArgumentPack, Keyword, Default = void> metafunction. Right now, when no 3rd argument is supplied indicating the default type, and the ArgumentPack contains no matching keyword, the result is void. Under the proposed change, the result would be a compile-time error. The upshot is that, to find out whether a particular parameter is in the ArgumentPack, you'd need to supply some tag type that you know won't appear in the ArgumentPack as the 3rd argument to binding, and check for that. Any objections? -- Dave Abrahams Boost Consulting www.boost-consulting.com