Inout unclearness

[Max Klimov via Digitalmars-d]

On Wednesday, 22 July 2015 at 07:49:07 UTC, Kagamin wrote:
>That's unsafe. Unsafe behavior should require a cast or 
>something like that.

Where is unsafety? Code is the same for 3 versions, it guarantees 
immutability of data because it was checked for const(T) and 
immutable(T).

> How about this?
>
> const(A) f(const(B) b);
> auto constBack(R,P)(const R r, P p);
>
> Then invoke:
>
> B b;
> A a = f(b).constBack(b);
>
> So `f` takes const parameters and returns const result, you 
> feed the result to the function `constBack` with one of 
> parameters and it infers constness from that parameter and 
> casts the result to the inferred constness.

Sorry, I didn't quite get it. What is this example for?