[BUG] sidecasts via is/as always fail

[gerlero]

Describe the bug
When attempting to sidecast (in a scenario with multiple inheritance) using the is/as syntax, all cast attempts fail—even when an equivalent dynamic_cast would succeed.

To Reproduce
Steps to reproduce the behavior:

1. Sample code - distilled down to minimal essentials please

struct B1 { virtual ~B1() = default; };

struct B2 { virtual ~B2() = default; };

struct D : B1, B2 {};

main: () -> int = {
    d: D = ();
    p: *B1 = d&;

    std::cout << p* is B2 << std::endl;
}

2. Command lines including which C++ compiler you are using
   x86-64 gcc 12.2 with -std=c++20 option

3. Expected result - what you expected to happen
   Program compiles and outputs 1 (i.e., p* is a B2)

4. Actual result/error
   Program compiles but prints 0

Additional context
The cpp2::is and cpp2::as overloads that would actually try a dynamic_cast in this situation are never considered because they are currently constrained to downcasts only (with is_base_of checks in their requires clauses).

EDIT: fix typo in code snippet

[hsutter]

Thanks @gerlero and @filipsajdak!

I think this is now fixed with #128, and I tried the example and it seems to be correct now.

[gerlero]

The is case seems to be fixed. However, the same sidecast with as is still failing.

Unfortunately, my example didn't explicitly cover that case (although replacing is->as in the snippet is enough; the cast will evaluate to nullptr even though it shouldn't). I can open a new issue for the as case if it helps with tracking.

[filipsajdak]

Yes, you are right. as will not work for sidecast.

In #106 I have covered handling bad as casts.

I have noticed that as for sidecast is missing - it requires bad cast handling. If the proposed solution for handling bad casts is accepted by @hsutter, I can provide a proper as cast.