Semantic action attached to a parser that has attribute type of `unused_type` shall not inhibit the outer rule's attribute composition

[saki7]

This kind of grammar definition does not alter the semantics of the internal attribute as it's unused_type in the first place. Therefore, there's no reason that we must inhibit the rule parser's attribute handling even when semantic action is attached.

constexpr auto some_rule_grammar_def = 
    lit("prefix").on_match([](auto&& ctx) {
        if (x4::get<some_local_variable>(ctx) == 42) {
            do_something();
            return true;
        }
        return false;
    }) >>
    body_parser
;

Fundamentally, the code above shall match the semantics below:

constexpr auto some_rule_grammar_def = 
    eps([](auto&& ctx) {
        if (x4::get<some_local_variable>(ctx) == 42) {
            do_something();
            return true;
        }
        return false;
    }) >>
    body_parser
;

Workaround

We can currently use x4::as<x4::unused_type>(...) to explicitly re-enable the inhibited attribute. However it's actually a hack (which I introduced for some good reason) when I implemented x4::as, so it's solely a workaround and should not be the desired way of expressing it in application code.

constexpr auto some_rule_grammar_def = 
    as<unused_type>(lit("prefix").on_match([](auto&& ctx) { // <----- as<unused_type>(...) added
        if (x4::get<some_local_variable>(ctx) == 42) {
            do_something();
            return true;
        }
        return false;
    })) >>
    body_parser
;

Additional info

Related: #100