alias this doesn't work for properties.

[John Colvin]

On Thursday, 1 August 2013 at 22:23:40 UTC, JS wrote:
> This is wrong because
>
> (cast(A)b.Y) = 3;
>
> works.

That's an interesting case. cast(A)b.Y) is using the B getter 
property, which makes sense as, due to the cast, nothing in B can 
be called for the assigment.

So you have (in psuedo-code, all properties expanded as 
getY/setY):
     b.getY.castToA = 3;
wheras the previous example is:
     b.setY(3);

Neither are an int being implicitly converted to an A


> b.Y = 3 should do the same as b.a = 3 which should be the same 
> as b.a.x = 3.

Why that can't work:

struct A
{
     double x;
     alias x this;
}
struct B
{
     private A a;
     @property A Y() { importantGetPrep(); return a; }
     @property void Y(A v) { importantSetPrep(); a = v; }
}
void main()
{
     B b;
     b.Y = 3;  //cannot be a call to the setter as cannot convert 
3 to A .
               //cannot be a call to the getter followed by an 
assigment
               //as the assignment would then have no effect on b.a
               //due to value semantics.
               //cannot be anything else as it would bypass the
               //important*etPrep that must be called on all 
accesses
               //to B.a .
}

Putting functions like important*etPrep in properties are one of 
the main reasons why properties are useful, it would be a very 
bad idea to let them be circumvented by an implementation change 
in an internal struct.