X3: variant support for container parsing

[Bockeman]

This code fails to compile:

#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/support/ast/variant.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

namespace x3		= boost::spirit::x3;
namespace ast {
  struct tagged_string	: std::string	{};
  enum token_enum	{lcomment, rcomment};
  struct tagged_token	{token_enum token;};
  struct comment_item	: x3::variant<tagged_string, tagged_token>
    {using base_type::base_type;using base_type::operator=;};
  struct comment	: std::vector<comment_item>{};
  struct start_item	: x3::variant<comment>
    {using base_type::base_type; using base_type::operator=;};
  struct start		: std::vector<start_item>	{using std::vector<start_item>::vector;};
}
BOOST_FUSION_ADAPT_STRUCT(ast::tagged_token,		token)
namespace grammar {
  x3::symbols<ast::token_enum>	lcomment_token;
  x3::symbols<ast::token_enum>	rcomment_token;
  x3::rule<struct lcomment,	ast::tagged_token>	const lcomment		= "lcomment";
  x3::rule<struct rcomment,	ast::tagged_token>	const rcomment		= "rcomment";
  x3::rule<struct comment,	ast::comment>		const comment		= "comment";
  x3::rule<struct start_rule,	ast::start>		const start_rule	= "start_rule";

  auto const lcomment_def	= lcomment_token;				// /*
  auto const rcomment_def	= rcomment_token;				// */
  
  // Compile fails:
  auto const comment_def	= lcomment >> *(x3::char_ - rcomment) >> rcomment;	// /* ... */

  // Compiles ok:
  //auto const comment_def	= lcomment >> (x3::rule<class id>{}=*(x3::char_ - rcomment)) >> rcomment;	// /* ... */

  auto const start_rule_def	= x3::no_skip[+comment];	// 1 ... ;
  
  BOOST_SPIRIT_DEFINE(lcomment, rcomment, comment, start_rule);
}
int main() {
  char const*			iter	= "/*c*/", * const end	= iter + std::strlen(iter);
  ast::start			ast;
  grammar::lcomment_token.add	("/*",		ast::lcomment);
  grammar::rcomment_token.add	("*/",		ast::rcomment);
  return !parse(iter, end, grammar::start_rule, ast) || iter!=end;
}

But if the text between the delimiters is wrapped as a separate rule, then it does compile.

This is a simplified version, where in reality tagged_string is not just a std::string but contains addition tagging information.

[Kojoley]

That's about variant support for containers parsing:

#include <boost/spirit/home/x3.hpp>
#include <boost/spirit/home/x3/support/ast/variant.hpp>
int main() {
    namespace x3 = boost::spirit::x3;
    char const* iter = "/*c*/", * const end = iter + std::strlen(iter);
    x3::variant<std::string, bool> x;
    x3::parse(iter, end, +x3::char_, x);
}