How to hint to GCC that a line should be unreachable at compile time?

It's common for compilers to provide a switch to warn when code is unreachable. I've also seen macros for some libraries, that provide assertions for unreachable code.

Is there a hint, such as through a pragma, or builtin that I can pass to GCC (or any other compilers for that matter), that will warn or error during compilation if it's determined that a line expected to be unreachable can actually be reached?

Here's an example:

    if (!conf->devpath) {        conf->devpath = arg;        return 0;    } // pass other opts into fuse    else {        return 1;    }    UNREACHABLE_LINE();

The value of this is in detecting, after changes in conditions above the expected unreachable line, that the line is in fact reachable.