difficulties with const structs and alias this / template functions

[Dennis]

On Monday, 19 November 2018 at 01:24:02 UTC, Stanislav Blinov 
wrote:
> Yup, that's because, like Rubn said, copying value types is 
> trivial. Where it all comes to bite you is when you start 
> having pointers, because you can't copy a const(T)* into a T*.

I'm not using reference types, but still:

```
struct S {
     int a;
     this(int a) {
         this.a = a;
     }
}

void main()
{
     immutable S d = 3;
}

```

onlineapp.d(10): Error: mutable method onlineapp.S.this is not 
callable using a immutable object
onlineapp.d(10):        Consider adding const or inout to 
onlineapp.S.this

const still leaves the first error, inout works though I don't 
know what it does.
Adding pure also works.