The checker resolves to no-op when used with variants.
```
template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
struct checker<boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> >
{};

template <BOOST_VARIANT_ENUM_PARAMS(typename T)>
struct checker<boost::variant<BOOST_VARIANT_ENUM_PARAMS(T)> const>
{};
```