X3: BOOST_SPIRIT_INSTANTIATE regression

[Xeverous]

after a3cf3f2 X3 requires existing code to change grammar to add more const x3::unused_type workarounds

This compiles fine just before a3cf3f2:

    #include <boost/spirit/home/x3.hpp>

    namespace x3 = boost::spirit::x3;

    using comment_type = x3::rule<class comment_class, /* const */ x3::unused_type>;
    BOOST_SPIRIT_DECLARE(comment_type)

    using identifier_type = x3::rule<class identifier_class, std::string>;
    BOOST_SPIRIT_DECLARE(identifier_type)

    using whitespace_type = x3::rule<class whitespace_class, /* const */ x3::unused_type>;
    BOOST_SPIRIT_DECLARE(whitespace_type)

    using grammar_type = x3::rule<class grammar_class, std::string>;
    BOOST_SPIRIT_DECLARE(grammar_type)

    using skipper_type = whitespace_type;
    using iterator_type = std::string::const_iterator;
    using context_type = x3::phrase_parse_context<skipper_type>::type;

    const comment_type comment = "comment";
    const auto comment_def = x3::lexeme['#' >> *(x3::char_ - '\n')];
    BOOST_SPIRIT_DEFINE(comment)

    const identifier_type identifier = "identifier";
    const auto identifier_def = x3::lexeme[(x3::alpha | '_') >> *(x3::alnum | '_')];
    BOOST_SPIRIT_DEFINE(identifier)

    const whitespace_type whitespace = "whitespace";
    const auto whitespace_def = x3::space;
    BOOST_SPIRIT_DEFINE(whitespace)

    const grammar_type grammar = "code";
    const auto grammar_def = identifier > comment;
    BOOST_SPIRIT_DEFINE(grammar)

    BOOST_SPIRIT_INSTANTIATE(grammar_type, iterator_type, context_type)

but after that commit, all grammars that do not syntesize any attribute have to specify const x3::unused_type (uncomment const)

Unfortunately the pull mentioned in https://stackoverflow.com/a/54095167/4818802 does not solve the problem, it only requires code to have more workarounds than previously.

[Kojoley]

X3 requires existing code to change grammar to add more const x3::unused_type workarounds

I do not see where you added more workarounds.

The problem seems that you have to remove const after the change and that was intended (actually you need to completely remove the third parameter). I did not add unused_type const specialization because of: 1) maintaining backward comparability with such workaround will make code more complex without much need 2) that workaround is fragile as it makes has_attribute == true.

The users that upgrade to a new version can just remove it.

Try to add this:

boost::spirit::x3::detail::rule_attr_deducer<unused_type const>
  : rule_attr_deducer<unused_type> {};

If it will work for you I will add it not to bother people with changing their code.

[Kojoley]

I tried the const workaround and it looks like it still works. Your code seems to uncover some other bug, will look into it.

[Kojoley]

Well, that's exactly what I meant to ban: if a parser with has_attribute==true assigns to a rule - that's look suspicious and there should be missing omit, but now I see that requiring omit is not practical.

[Kojoley]

Hmm, something strange happens here, I will need more time to dig into this. BOOST_SPIRIT_INSTANTIATE(grammar_type, iterator_type, context_type) somehow leads to instantiation of parse_rule for whitespace_type with std::string attribute, I do not understand how it happens.

[cppljevans]

On 2/6/19 6:47 PM, Nikita Kniazev wrote: Hmm, something strange happens here, I will need more time to dig into this. `BOOST_SPIRIT_INSTANTIATE(grammar_type, iterator_type, context_type)` somehow leads to instantiation of `parse_rule` with `std::string` attribute, I do not understand how it happens.

I think because grammar_type::attribute_type **is** std::string and, according to what I see on my checkout of that commit, that's exactly what BOOST_SPIRIT_INSTANTIATE does: #define BOOST_SPIRIT_INSTANTIATE(rule_type, Iterator, Context) template bool parse_rule < Iterator, Context, rule_type::attribute_type //std::string (... So where's the mystery?

[Kojoley]

I edited my message because I missed "for whitespace_type". Run the code or open GitHub to see it.

[Kojoley]

I think this happens because of ADL/overload resolution. I was thinking previously why parse_rule is not hidden under rule itself, looks like it was a useful idea.

[Xeverous]

I tried the const workaround and it looks like it still works. Your code seems to uncover some other bug, will look into it.

Yes, the workaround still works but now I have to place const x3::unused_type in more places. Code without any unused type in templates does not compile.

Removing all const x3::unused_type and x3::unused_type does not solve the issue - try with the code I posted.