const AA require

[jfondren]

On Wednesday, 4 August 2021 at 08:08:35 UTC, Tejas wrote:
> On Thursday, 29 July 2021 at 20:44:35 UTC, Elronnd wrote:
>> const(int)[int] d;
>> d.require(5, 7); //compile error
>>
>> From a semantics perspective, though, I don't think there's 
>> anything wrong with this, and 'require' could reasonably cast 
>> the const away before assigning.  Am I missing anything?
>
> Casting ```const``` away is undefined behaviour in D. And what 
> is this "require"? Maybe show more code?

what's wanted here is an assign-once hash table, I imagine.

require is https://dlang.org/phobos/object.html#.require , and 
it'd be the assignment line of that code that would have to cast 
away const.

```d
unittest {
     int[int] nums;
     nums.require(0, 0);
     nums[0]++;
     assert(nums == [0: 1]);
}

unittest {
     const(int)[int] nums;
     assert(__traits(compiles, nums.require(0, 0)));
     assert(!__traits(compiles, nums[0]++));
}
```