Why is std::vector::at not working in the for-each loop using ranges?

This is a reduced example what I try to achieve for a compile-time parser in C++26 (I know, not released, but that's the option I give to gcc) and GCC 14.2.

#include <iostream>
#include <ranges>
#include <string_view>
#include <vector>

using namespace std::string_view_literals;

template <class To>
inline constexpr auto static_caster = []<class From>(From &&from) -> To
    requires requires { static_cast<To>(std::declval<From>()); }
{ return static_cast<To>(std::forward<From>(from)); };

struct Test {
    bool value;
    constexpr Test(const std::string_view input) {
        const std::vector<std::string_view> lines =
            input | std::views::split('\n') |
            std::views::transform(static_caster<std::string_view>) |
            std::ranges::to<std::vector>();
        // const std::string_view line = lines.at(0);
        for (const std::string_view line : lines) {
            const std::vector<std::string_view> line_tokens =
                line | std::views::split(' ') |
                std::views::transform(static_caster<std::string_view>) |
                std::ranges::to<std::vector>();
            const std::string_view kind = line_tokens.at(0);
            this->value = "v" == kind;
        }
    }
};

int main(int argc, char *argv[]) {
    constexpr bool value = Test("v 1.0 2.0 3.0\n"sv).value;

    std::cout << value;

    return 0;
}

Error message:

In file included from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/vector:68,
                 from <source>:4:
/opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/stl_vector.h: In function 'int main(int, char**)':
<source>:33:52:   in 'constexpr' expansion of 'Test(std::literals::string_view_literals::operator""sv(((const char*)"v 1.0 2.0 3.0\012"), 14))'
<source>:26:57:   in 'constexpr' expansion of 'line_tokens.std::vector<std::basic_string_view<char> >::at(0)'
/opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/stl_vector.h:1333:16:   in 'constexpr' expansion of '((const std::vector<std::basic_string_view<char> >*)this)->std::vector<std::basic_string_view<char> >::_M_range_check(__n)'
/opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/stl_vector.h:1292:35: error: call to non-'constexpr' function 'void std::__throw_out_of_range_fmt(const char*, ...)'
 1292 |           __throw_out_of_range_fmt(__N("vector::_M_range_check: __n "
      |           ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1293 |                                        "(which is %zu) >= this->size() "
      |                                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1294 |                                        "(which is %zu)"),
      |                                        ~~~~~~~~~~~~~~~~~~
 1295 |                                    __n, this->size());
      |                                    ~~~~~~~~~~~~~~~~~~
In file included from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/new_allocator.h:35,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/x86_64-linux-gnu/bits/c++allocator.h:33,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/allocator.h:46,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/string:45,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/locale_classes.h:42,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/ios_base.h:43,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/ios:46,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/ostream:42,
                 from /opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/iostream:43,
                 from <source>:1:
/opt/compiler-explorer/gcc-trunk-20241219/include/c++/15.0.0/bits/functexcept.h:82:3: note: 'void std::__throw_out_of_range_fmt(const char*, ...)' declared here
   82 |   __throw_out_of_range_fmt(const char*, ...) __attribute__((__noreturn__,__cold__))
      |   ^~~~~~~~~~~~~~~~~~~~~~~~

This example is not working, but the one with the inner for-loop only having the content of it used together with a fixed line at index 0 works.

Interestingly it works when I remove the \n from the constant.

The question is why and how can I solve that?

The problem has nothing to do with fancy C++26 stuff; it is a simple (but easy to miss) bug. If you end with \n and split on \n you will get an empty line. That empty line will contain no tokens. And empty vectors don't have a '0th' element.

To fix this you can add std::views::filter([](auto&& x){return !x.empty();}) | in the first transformation sequence before the transformation to std::vector to filter out empty lines, as they don't have any tokens.

Edit: I also recommend filtering out empty tokens in the same way in the loop.