Since you're writing a library, you don't want to replace the default handler.
IIRC the way to have a contract use a specific handler is
by passing it as the first template argument,
e.g. assert<my_handler>(a && b);.

void test() {
 throw std::runtime_error("message to retrieve");
}
main:() -> int = {
 test();
 return 0;
}

I can't figure out what to write in the main function to get my error message displayed at runtime.
(Assuming that the test method comes from an imported cpp1 library.)

Sounds like you need try/catch syntax in Cpp2, which isn't implemented yet.

Ah, I have the impression that this is a CMake and compiler option related problem more than a cpp2 problem. Sorry for the inconvenience and thanks for the help. I'll keep working on a solution on my own.

I've found the perfect simple solution, just encapsulate all the code in a try catch 😊

cpp2_main:() -> int = {
 // My entire cpp2 code.
}
int main() {
 try { // CPP2 workaround: Try catch not yet supported.
 return cpp2_main();
 } catch (const std::exception& e) {
 std::cerr << "Caught exception: " << e.what() << std::endl;
 }
 return 0;
}