Undefined reference to index_tag<0ul> on clang 17

[nikhilkalige]

clang above v17 throws an error during linking.

int main() {
    nonstd::variant< char, int, long, std::string > var;
    var =  'v' ;
    return 0;
}

/opt/compiler-explorer/gcc-13.2.0/lib/gcc/x86_64-linux-gnu/13.2.0/../../../../x86_64-linux-gnu/bin/ld: /tmp/example-480496.o:(.debug_addr+0x20): undefined reference to `void nonstd::variants::detail::index_tag<0ul>(nonstd::variants::detail::index_tag_t<0ul>)'

I compiled with clang 16 which works and compare the output of the object files from clang 17. The only different between the two is below

Clang-16 0000000000000000 W void nonstd::variants::detail::index_tag<0ul>(nonstd::variants::detail::index_tag_t<0ul>)
Clang-17                  U void nonstd::variants::detail::index_tag<0ul>(nonstd::variants::detail::index_tag_t<0ul>)

https://godbolt.org/z/fcY571qs7

[martinmoene]

Thanks @nikhilkalige for your report.

No idea yet how to tackle this.

[geghamyan]

Here is the PR that fixes the issue: #56
I simplified the non-type template parameter by replacing the reference (which unfortunately generates an undefined reference) to a function with a pointer. Another option would be to use conditional compilation and apply the pointer only for Clang, but that seems like unnecessary complication. I also fixed the tests that were failing with GCC and Clang.

[geghamyan]

@martinmoene could you please check the result of script/generate_header.py? It looks like the output it generates is out of sync with the current implementation in template/variant.hpp. I suspect there were changes made to include/nonstd/variant.hpp, but the template was not updated accordingly.

[martinmoene]

Thanks for your help. Hope to look into it later today / tonight.

[martinmoene]

@nikhilkalige There is indeed one commit that changes include/nonstd/variant.hpp instead of template/variant.hpp.

Clearly I forgot about the update script :(

See also this remark at issue 55.

[geghamyan]

Sounds good, thank you. Please let me know if you'd like me to make any updates to the PR. Thank you for supporting this project.

[martinmoene]

Merged PR #56.