Class ComposableParserImpl

tests/catch.hpp:7125–7132

    template<typename DerivedT>
    class ComposableParserImpl : public ParserBase {
    public:
        template<typename T>
        auto operator|( T const &other ) const -> Parser;

		template<typename T>
        auto operator+( T const &other ) const -> Parser;
    };

    // Common code and state for Args and Opts
    template<typename DerivedT>