Alias this does not work with pointers?

[Chris Wright via Digitalmars-d]

alias this can do so much more, though.

For instance:

  struct Foo {
    int a;
    string b;
    alias a this;
  }
  void bar(int* p, size_t len) {
    for (int i = 0; i < len; i++) {
      p[i] = 0;
    }
  }
  bar(new Foo[2].ptr, 2);

Two fields. You don't want to overwrite a pointer, so you can't just turn 
it to an implicit cast.

Or another example:

  struct Foo {
    double s;
    int square() { return cast(int)(s * s); }
    alias square this;
  }

How do you convert a Foo* to an int*? Do you allocate space for it on the 
heap? But what if the Foo* points to a malloc'd section of memory? Or 
some other allocator? Or if it's on the heap and the function is marked 
@nogc? You could put it on the stack as an alternative, but then the 
pointer will be invalid when the function exits... What if it's sent off 
to a C function that stores it? Should the allocation be pinned by 
default? When would it be unpinned?

It's a silent, hidden allocation beyond the user's control. We would like 
it to be allocated like the Foo* was allocated, but that's not currently 
possible.

Instead of providing an implementation that occasionally works but 
typically causes a crash or an unavoidable inefficiency, it's just not 
there to trip people up.