Since: http://www.boost.org/doc/libs/1_65_0/libs/spirit/doc/html/spirit/qi/quick_re... doesn't mention a unary variant: variant<A> and the comments in: https://github.com/boostorg/spirit/blob/develop/include/boost/spirit/home/qi... indicates variant<T> is collapsed to T, why isn't the x3::variant here: https://github.com/boostorg/spirit/blob/develop/include/boost/spirit/home/x3... declared as: template <typename L, typename R, typename ...Types> struct variant instead of: template <typename ...Types> struct variant which allows sizeof...(Types)==0 instead of the minimum size of 2? That would disallow the primary and factor using's here: https://stackoverflow.com/questions/43791079/x3-linker-error-with-separate-t... which make no sense because, for example, primary_def is *not* an alternative parser. -regards. Larry