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[Timon Gehr via Digitalmars-d]

On 11/15/2015 10:06 PM, Andrei Alexandrescu wrote:
> ...
> I do agree that "pure const" may have other, stronger
> properties.  But let's discuss "const" and what it can promise, or move
> over to a whole new topic of "pure const".
> ...

"pure" is independent of "const" at the moment. So what you want to 
discuss is "const" in impure method signatures. Fine.

In any case, consider that you might at some point be asked to explain 
why a const List is a good thing to want but a const pure List isn't.

>>> Note that I
>>> could even afford to have getFoo() return const, so no need for the
>>> caller to make it so!
>>>
>>> With immutable, it's all cool. Immutable data is truly immutable, and
>>> that can be counted on. But const, even today, cannot be assumed to be
>>> as strong.
>>> ...
>>
>> This is obviously true (it is what justifies the distinction between
>> const and immutable in the first place), but this is not a way to
>> justify making it weaker.
>
> Weaker than what? I've just shown black on white it's not a guarantee of
> constant data.

In one particular case.

import package.module : foo, bar, baz, fun;

final class C{
     int x;
     private this(int x){ this.x=x; }
}

void main(){
     auto c = new C(2);
     foo(c); // void foo(const C c);
     bar();  // impure
     baz();  // impure
     fun(c.x); // this read of c.x can be optimized away under current 
semantics, not under the new ones
}


> Today. With no compiler bugs in sight.
> ...

Do you mean, even ignoring compiler bugs?

>> This is all just moving in the direction of a
>> setting where all structs/classes just prevent immutable construction
>> and make all member functions const, and const becomes the new mutable.
>> I don't see the point.
>
> I don't understand this.
>...

const is transitive and contagious. Make casting away const legal and 
spuriously require const in some places in the library, and you will 
have created a quite large incentive to use the now legal casts in a way 
that some will consider unprincipled.