id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
7730	Generic specializations of is_nullary for custom terminals are not possible	Andrey Semashev	Thomas Heller	"The is_nullary trait is specialized for all custom_terminal<T> to be true (see phoenix/code/is_nullary.hpp).

{{{
template <typename T>
struct is_nullary<custom_terminal<T> >
    : mpl::true_
{};
}}}

This is not true with regard to multiple terminals I define in Boost.Log. This forces me to specialize is_nullary for all my custom terminals, and I cannot provide a single blanket specialization for all my terminals.

The is_nullary trait has a second template parameter which is intended to be used exactly for this purpose. A nested tag void typedef can be used to match the trait for a set of types. I could create the following specialization:

{{{
template <typename T>
struct is_nullary<custom_terminal<T>, typename T::_is_my_terminal >
    : mpl::false_
{};
}}}

However this extension mechanism does not work because the two specializations are considered equally specialized and the compiler reports ambiguity.

I suggest to limit the first specialization to only match the default custom terminals, e.g.:

{{{
template <typename T>
struct is_nullary<custom_terminal<T>, typename custom_terminal<T>::_is_default_custom_terminal >
    : mpl::true_
{};
}}}

Where typedef void _is_default_custom_terminal will be added to the generic custom_terminal template.
"	Bugs	new	To Be Determined	phoenix	Boost 1.52.0	Problem