List separated by two newlines

[codergal6502]

Hi. I'm trying to parse a list separated by two newlines. In the sandbox, I wrote this example:

struct numbers
{
    static constexpr auto rule = dsl::list(dsl::integer<uint32_t>(dsl::digits<dsl::hex>), dsl::sep(LEXY_LIT(" ")));
};

struct complicated
{
    static constexpr auto rule = LEXY_LIT("line1") + LEXY_LIT(" ") + dsl::p<numbers> + dsl::newline + LEXY_LIT("line2");
};

struct production
{
    static constexpr auto rule = dsl::list(dsl::p<complicated>, dsl::sep(dsl::newline));
};

That parses this successfully:

line1 04 08 0A
line2
line1 01
line2
line1 01 02 03
line2

However, when I switch to trying to use two newlines:

struct numbers
{
    static constexpr auto rule = dsl::list(dsl::integer<uint32_t>(dsl::digits<dsl::hex>), dsl::sep(LEXY_LIT(" ")));
};

struct complicated
{
    static constexpr auto rule = LEXY_LIT("line1") + LEXY_LIT(" ") + dsl::p<numbers> + dsl::newline + LEXY_LIT("line2");
};

struct production
{
    static constexpr auto rule = dsl::list(dsl::p<complicated>, dsl::sep(dsl::newline + dsl::newline));
    //                                                                     added this ^^^^^^^^^^^^^^^
};

I then get an error:

<source>:19312:5: error: static assertion failed due to requirement 'lexy::_require_branch_rule<lexyd::_seq<lexyd::_nl, lexyd::_nl>>': sep requires a branch condition. You may need to use `>>` to specify the condition that is used for dispatch. See https://lexy.foonathan.net/learn/branching/ for more information.

Reading https://lexy.foonathan.net/reference/dsl/newline/#newline, as well as the documentation for LEXY_LIT, they're both token rules. Is that why this happens? Is something else going on? In my non-toy example, I can probably work around this, but I am curious for future reference. Thanks!

[foonathan]

A sequence rule is no longer a branch rule, because it would involve non-trivial backtracking. If you want two newlines as separators, you need something like LEXY_LIT("\n\n") | LEXY_LIT("\r\n\r\n") (and potentially also \r\n\n or \n\r\n if you want to be super lenient).