D does have head const (but maybe it shouldn't)

[Petar]

On Tuesday, 29 December 2020 at 16:15:56 UTC, ag0aep6g wrote:
> Exercise: Consider the following `main` function. Define `f` 
> and `g` so that the code is valid, compiles, and runs 
> successfully. The asserts must be executed and they must pass.
>
> ----
> void main() pure @safe
> {
>     int* x = new int;
>     const y = f(x);
>     *x = 1;
>     g(y);
>     assert(*x == 2);
> }
> ----
>
> Hints:
> * You can't store `x` in a global because the code is `pure`.
> * You can't cast `const` away in `g` because that would be 
> invalid.
> * You're supposed to find a type for which `const` means head 
> const.
>
> Solution: https://run.dlang.io/is/dsaGFS
>
> The validity of the given solution might be arguable, and I'd 
> be in favour of outlawing it. It's surprising that there's a 
> type for which `const` means head const when it means 
> transitive const for everything else. It's so surprising that 
> even DMD trips over it 
> (<https://issues.dlang.org/show_bug.cgi?id=21511>).

Wow. The solution surprised me, even though in retrospect it's 
obvious to me why it works (bugs in this area of the type system 
have been know for more than a few years). I'd say head-const is 
a very useful tool, but it's pretty obvious to me that this 
shouldn't be a supported way to implement it.