Why struct opEquals must be const?

[Steven Schveighoffer]

On Sun, 17 Oct 2010 02:57:02 -0400, Nick Sabalausky <a at a.a> wrote:

> Is there a technical reason why the l- and r- values for opEquals must be
> const? If the restriction is purely for the intuitive notion that  
> there's no
> heisenstructs, then I have an example I think might be worth  
> consideration:
> lazy caching.
>
> If comparison isn't always needed and requires a potentially expensive
> computation that isn't likely needed otherwise (for example, a wrapper  
> for
> string that does case-insensitive comparisons, if it's used in a  
> situation
> that does more assigning/slicing/etc than comparing), then it may make  
> sense
> to wait until an opEquals is called, and then compute the information and
> cache it. But that requires mutating state and so can't be done in a  
> struct
> opEquals.
>
> 'Course, if there is a technical reason for the restriction, then all  
> this
> is moot.

http://d.puremagic.com/issues/show_bug.cgi?id=3659

-Steve