the error comes from your code, you removed the const qualifier from the input (std::string &s instead of const std::string &s). As a result, the tests cannot find your function. this is a linker error, unrelated to the warning for comparison of signed and unsigned integers.

sorry is that a problem with the kata or your own code? what are you saying that the kata should do differently? all I see from your post is that you point out a problem with your own code.

also, when you say "treated as error" I do wonder if you mean that it's showing you stderr, as opposed to refusing to compile, because your output says warning, not error, and if I produce warnings in the c++ env here then it does not treat that as an error - which is to say that it displays it but still compiles

Code ran with warning on local and online compiler with Clang++8.0.1 (was Clang++ freaks out), but that was enough to be treated by Codewars as an error:

In file included from main.cpp:6:
./solution.cpp:16:7: warning: variable 'i' is incremented both in the loop header and in the loop body [-Wfor-loop-analysis]
i++;
^
./solution.cpp:12:35: note: incremented here
for (int i = 0; i < s.size(); i++) {
^
./solution.cpp:12:23: warning: comparison of integers of different signs: 'int' and 'std::__cxx11::basic_string<char, std::char_traits, std::allocator >::size_type' (aka 'unsigned long') [-Wsign-compare]
for (int i = 0; i < s.size(); i++) {
~ ^ ~~~~~~~~
2 warnings generated.
/tmp/main-1368cb.o: In function do_test(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&)': main.cpp:(.text+0x15): undefined reference to solution(std::__cxx11::basic_string<char, std::char_traits, std::allocator > const&)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)