alias this doesn't work for properties.

[Jesse Phillips]

I'm going to provide a reduced case for the issue you are having.

struct Foo {
     double x;
     alias x this;
}

void main() {
     Foo a;
     a = 8; // Alias this assignment
     assert(a.x == 8);
     fun(7); // What you want
}

void fun(Foo a) {
}

You're claiming that since Foo is requested to "be" a double 
calling fun(7) should just assign 7 to x.

What you're missing is that it is Foo which behaves like double, 
however fun(7) there is no Foo involved, it doesn't exist. You're 
actually requesting that a Foo is created, then to have an 
assignment of 7 to x of the new temporary Foo.

This is not a bug, it is by design. Alias this is sugar of type 
it is declared in not the type it is declared to.