`Alias this` to a mixed in property

[Bastiaan Veelo]

Hi,

`Alias this` to mixed in properties does not seem to work, see 
below. If you think it should, I'll file an issue. Otherwise: can 
this be made to work somehow?

Interestingly, if you uncomment either the mixin getter or setter 
(row 36 or 37) and its corresponding use in `main`, then the 
remaining property works. Does the compiler detect identical 
names and then does some additional mangling which messes with my 
`alias this` maybe?

```
import std.stdio;

// version = manual; // Manually written property works, 
obviously.

mixin template getter()
{
     @property int p()
     {
         writeln(__LINE__, " mixin getter");
         return 3;
     }
}
mixin template setter(T)
{
     @property int p(T arg)
     {
         writeln(__LINE__, " mixin setter ", typeid(T), ' ',  arg);
         return 4;
     }
}
struct S
{
     version (manual) {
         @property int p()
         {
             writeln(__LINE__, " manual getter");
             return 3;
         }
         @property int p(int arg)
         {
             writeln(__LINE__, " manual setter int ",  arg);
             return 4;
         }
     }
     else {
         mixin getter;            // row 36
         mixin setter!int;        // row 37
     }
     alias p this;
}


void main(string[] args)
{
     S s;
     s = 7;
     int i = s;
}
```

Error: cannot implicitly convert expression s of type S to int.