Support formatting nested exceptions

[Krzmbrzl]

As far as I can tell, support for formatting regular exceptions has been added via #3062 and this was then extended by #3076.
However, this implementation is currently missing support for nested exceptions (see e.g. https://cplusplus.com/reference/exception/nested_exception/ and https://riptutorial.com/cplusplus/example/18879/nested-exception).

When dealing with nested exceptions, simply calling what() only reveals the message of the topmost exception, but all messages from the nested exceptions (which are typically more interesting) are ignored. In order to print all messages, something along the line of this is typically used (source)

// recursively print exception whats:
void print_what (const std::exception& e) {
    std::cerr << e.what() << '\n';
    try {
        std::rethrow_if_nested(e);
    } catch (const std::exception& nested) {
        std::cerr << "nested: ";
        print_what(nested);
    }
}

It'd be great if formatting an exception with this library would also print the messages of the nested exceptions in order to save users the hassle of having to take care of this themselves.

However, since implementing this requires rethrowing, I guess this is incompatible with builds that disable exceptions altogether (which, as I understand it, is a support case for this library). I guess there has to be some preprocessor magic to remove this feature in builds without exceptions.

Godbolt: https://godbolt.org/z/Yc3KsY193 (currently fails to compile as apparently the version of fmt that is used doesn't support exceptions yet)

[Skylion007]

However, since implementing this requires rethrowing, I guess this is incompatible with builds that disable exceptions altogether (which, as I understand it, is a support case for this library). I guess there has to be some preprocessor magic to remove this feature in builds without exceptions.

it actually doesn't require rethrowing necessarily, but it may require RTTI otherwise. (Dynamic cast to nested exception and then get the exception ptr of the nested exception).

[ShawnZhong]

I can successfully get an exception_ptr from a nested exception, but there is not much you can do with it other than calling rethrow_exception on it.

If you are comfortable with casting exception_ptr to a std::exception * (which is definitely UB), then the following code snippet would work:

void print_exception(const std::exception &e) {
  fmt::print("{}\n", e.what());
  if (auto nep = dynamic_cast<const std::nested_exception *>(&e)) {
    auto ptr = nep->nested_ptr();
    print_exception(**(std::exception **)(&ptr));
  }
}

[vitaut]

Closing since it is possible to do this via the usual extension API but a PR to add this to the existing formatter would be welcome too.