case_folding discards errors in validate and parse actions

If an error occurs during parsing with case folding the error is discarded and the return value from validate and parse is incorrect.

Example of grammar on playground: https://lexy.foonathan.net/playground/?id=fccxs48G3&mode=trace

namespace grammar
{
constexpr auto kw = dsl::identifier(dsl::ascii::alpha, dsl::ascii::alpha_underscore / LEXY_LIT("-"));

constexpr auto kw_create = dsl::ascii::case_folding(LEXY_KEYWORD(u8"create", kw));

struct create_number
{
    static constexpr auto whitespace = dsl::ascii::space;
    static constexpr auto rule = kw_create + dsl::integer<std::size_t>(dsl::n_digits<3>) + dsl::eof;
    static constexpr auto value = lexy::as_integer<std::size_t>;
};

}

int main()
{
    using namespace std::string_literals;
    for (const auto line : { "create 123"s, "creat a"s })
    {
        lexy::string_input<lexy::utf8_char_encoding> input{line};

        if (auto result{lexy::parse<grammar::create_number>(input, lexy_ext::report_error)})
        {
            std::cout << "has_value(): " << result.has_value() << '\n';
            std::cout << "is_success(): " << result.is_success() << '\n';
            std::cout << "is_error(): " << result.is_error() << '\n';
            std::cout << "is_recovered_error(): " << result.is_recovered_error() << '\n';
            std::cout << "is_fatal_error(): " << result.is_fatal_error() << '\n';
            std::cout << "error_count(): " << result.error_count() << '\n';
            std::cout << '"' << line << "\" evaluates to " << result.value() << '\n';
        }
        else
        {
            std::cout << "fail";
        }
    }
}

The output of this program is:

has_value(): 1
is_success(): 1
is_error(): 0
is_recovered_error(): 0
is_fatal_error(): 0
error_count(): 0
"create 123" evaluates to 123

has_value(): 0
is_success(): 1
is_error(): 0
is_recovered_error(): 0
is_fatal_error(): 0
error_count(): 0

Afterwards it crashes on the following assertion:

lexy_example: /mnt/work/git/notes.txt/libraries/lexy/build/_deps/lexy-src/include/lexy/_detail/lazy_init.hpp:145: constexpr const T& lexy::_detail::lazy_init<T>::operator*() const & [with T = long unsigned int]: Assertion `*this' failed.

String, create 123 is parsed as expected, but the result of parsing the other one creat a is strange, it says it successful but it doesn't have a value. I would expect that it reports that the second string wasn't parsed.

If i remove dsl::ascii::case_folding from kw_create then the second string isn't parsed and the error is reported.

EDIT: I've debugged this problem further figured the following. This is caused by overloads of on method in class event_handler:

        constexpr void on(_vh&                                   handler, parse_events::error,
                          const error<Reader, expected_literal>& error)
        {
            handler._cb.literal(handler._cb.sink, get_info(), handler._cb.input, _begin, error);
        }
...
        template <typename Event, typename... Args>
        constexpr auto on(_vh&, Event, const Args&...)
        {
            return 0; // operation_chain_start must return something
        }

When parsing a case folding rule, the real Reader gets wrapped into a case_folding one and it doesn't match the Reader template parameter anymore, so the template overload gets called which doesn't do anything. This is the instantiated method from gdb: lexy::_vh<lexy::_prc>::event_handler::on<lexy::parse_events::error, lexy::error<lexy::_acfr<lexy::_prc>, lexy::expected_literal> >

foonathan / lexy

case_folding discards errors in validate and parse actions #149