Why do "const inout" and "const inout shared" exist?

[Stefan Koch via Digitalmars-d]

On Saturday, 1 July 2017 at 22:16:12 UTC, Timon Gehr wrote:
>
> struct S{
>     int x;
>     ref inout(int) foo()inout{
>         return x;
>     }
> }
>
> void main(){
>     S s;
>     s.foo()++; // ok!
>     const(S) t = s;
>     import std.stdio;
>     writeln(t.foo());
>     // t.foo()++; // error
> }

Oh damn. I was not aware that it could behave non-constly.
since when does it do that ?