Subtyping with "alias this" doesn't mix with regular inheritance (issues 5380)

[Stanislav Blinov]

Copied from bugzilla:

--- Comment #0 from Max Samukha <samukha at voliacable.com> 2010-12-27 
03:59:11 PST ---
class A
{
}

class B
{
     A a;
     alias a this;
}

class C : B
{
}

void main()
{
     A a = new C; // error
}

Error: cannot implicitly convert expression (new C) of type test.C to test.A

Since C is a subtype of B (via inheritance) and B is a subtype of A (via 
"alias
this"), C should be implicitly convertible to A.

--- Comment #1 from Walter Bright <bugzilla at digitalmars.com> 2010-12-27 
13:33:28 PST ---
Yes, I see the problem. Never thought of that.


---

I must say after a long thought I *may* see the problem too, but...

Would you guys mind explaining what should this mean? How is it inferred 
that C should be "implicitly" convertible to A in this case? I see a way 
of substituting the *existing* reference, but construction? What if B or 
C aren't visible during compilation (e.g. are introduced later in a 
separate shared library)?