implicit conversion to alias this

[Tobias Pankrath]

---------
struct A { bool a; alias a this; }
struct B { int b; alias b this; }

A a = false; // works
B b = 12; // works

struct C
{
     A aa;
     B ab;
}

C c = { false, 12 }; // does not work, because the implicit 
conversion does not happen.
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What is the reason to allow the first two assignments? Isn't it 
just another implicit conversion that shouldn't be allowed?