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

[ag0aep6g]

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>).