Why can't function expecting immutable arg take mutable input?

[Mike Parker via Digitalmars-d-learn]

On Friday, 16 October 2015 at 10:35:23 UTC, Shriramana Sharma 
wrote:
> Hello. I still haven't wrapped my mind around the 
> const/immutable thing yet and am still stuck in C/C++ mode. :-(
>
> A function that takes mutable arguments cannot be called with 
> immutable input at the call site since it does not promise to 
> *not* mutate the input. That's of course clear.
>
> Why can't a function that takes an immutable argument be called 
> with a mutable input at the call site?
>
>

The contract of immutable is such that any reference to immutable 
data is a guarantee that no reference to the same data will 
modify it anywhere in the program. Passing mutable data to a 
function where an immutable parameter is declared would break 
that contract, since the data could be modified through the 
original reference. The compiler can take advantage of such a 
strict contract to make optimizations it would otherwise be 
unable to.

const only guarantees const data will not be modifed through a 
single reference, but says nothing about other references to the 
same data. A const parameter can accept const, immutable, and 
mutable arguments.