[BUG] sidecasts via is/as always fail

[gerlero]

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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