[spirit] bad template error when complementing a rule

[Eric Niebler]

The following code results in a terrible compiler error. The grammar for Spirit should not allow ~ to apply to a rule object.

#include <boost/spirit/home/qi.hpp>
 
int main()
{
    using namespace boost::spirit::qi;
    rule<char const *> expression, term, factor;
 
    expression  = term >> *( ( '+' >> term ) | ( '-' >> term ) ) ;
    term        = factor >> *( ( '*' >> ~factor ) | ( '/' >> factor ) ) ;
    factor      = uint_ | '(' >> expression >> ')' | '-' >> factor ;
}

[Hartmut Kaiser]

Well, the error you're seeing is exactly caused by the ~, so in a sense the grammar doesn't allow using it, no?

[Eric Niebler]

If the grammar disallowed it, the error message would have been short and meaningful, caught in rule::operator=. Instead, the error happens deep in Spirit's meta-guts. At least it did the last time I checked.

If the error doesn't bother you, or if it's too much trouble to improve it, or if it would too severely affect compile times, then yes, close this bug. But don't close it because you think the grammar correctly handles this case. I don't think it does.

[Joel de Guzman]

Eric has a valid point. I'll reopen this ticket and try to fix this.

[Joel de Guzman]

This has long since been fixed. Now we get "Static_assert failed "subject_is_not_negatable".