The problem was with copy/move constructors of boost::optional<T>, which invoked
optional_base<T> constructors with a single argument. Since optional_base has
copy/move constructors along with initializing constructors taking a single
argument, the latter may be considered by the compiler for viability. While
doing so, the compiler may instantiate the template constructor of T with
an argument of optional_base<T>, which in turn may fail if the constructor
attempts to inspect the type of its argument (e.g. to constrain the set of
acceptable types). Specifically, this happens with clang in C++03 mode,
when boost::multiprecision::number is wrapped in boost::optional and
a copy constructor of boost::optional is invoked.
This commit fixes the problem by destinguishing copy/move constructors of
optional_base from initializing constructors with an additional tag argument.
