I was trying to make autodiff (https://autodiff.github.io/installation/) using cmake and the most recent version of Catch2 on Cygwin, but would always run into an error at the cmake --build . --target install. I finally narrowed down my issue thanks to this error message :

/usr/local/include/catch2/catch_tostring.hpp: In static member function ‘static std::string Catch::StringMaker<char [SZ]>::convert(const char)’:
/usr/local/include/catch2/catch_tostring.hpp:214:33: error: there are no arguments to ‘strnlen’ that depend on a template parameter, so a declaration of ‘strnlen’ must be available [-fpermissive]

I think the issue is that in the file catch_tostring.hpp there is a call to 'strnlen' that is unfortunately not supported in Cygwin (microsoft/GSL#331).

I have tried to debug my issue by editing this file and changing the strnlen calls.

I understand there are several limitations to using cygwin and I am posting this here in case anyone else has this issue. Please let me know if this is not a bug and there was an issue in my implementation.