reference_wrapper & implicit conversion to parent types

[Christopher <kemsleyc@…>]

I propose a change to reference_wrapper which enables implicit conversion to references and reference_wrappers of base types.

One motivation for this is an unconventional use of reference_wrapper:

Many common library functions in code have a non-trivial set of arguments, some of which being non-const references. When reading code that uses these functions, it is often difficult to tell what side-effects a library function has from call to call. Having a coding policy that mutable references must be reference_wrappers and having calling code use ref() enables readers of the callsites to clearly identify which arguments have the possibility of having side-effects.

However, this is currently not possible without creating a custom implementation of reference_wrapper.

I have attached a proposed modification to the boost 1.55.0 implementation, as well as a simple file testing the extended capability.