What does this do, turn a specific warning into an error? Can this be filtered out (which would appear to be more consistent) rather than included?

What does this do, turn a specific warning into an error?

Yes.

Can this be filtered out (which would appear to be more consistent) rather than included?

Possibly, though I don't know how far to take it.

In this particular example, the flag was relied on for feature detection. CMake was being tested to use if the following code compiled:

#include <libunwind.h>
int main(int argc, char **argv) {
  unw_addr_space_t as;
  unw_get_accessors(as);
  return 0;
}

It was in particular testing for unw_get_accessors's existence. Because C is a weird language, if unw_get_accessors doesn't exist then this still actually compiles (with a warning) and fails while linking or potentially at runtime.

They relied on -Werror=implicit-function-declaration to instead make the warning an error and fail compile.

I could just do implicit-function-declaration and add to it as I find more. It's a little tricky to spot when it happens but I know now to look out for false feature detection.

I could just do implicit-function-declaration and add to it as I find more. It's a little tricky to spot when it happens but I know now to look out for false feature detection.

Yeh, this seems like a good call 👍

Thanks again @Bo98!