deep copying a struct

[Paul Backus]

On Friday, 6 September 2019 at 10:52:43 UTC, aliak wrote:
> On Friday, 6 September 2019 at 10:37:16 UTC, aliak wrote:
>> Are there any library APIs that allow this:
>
> I just put this together. Any holes other the AA related ones?

string a = "hello";
string b = a.dupDeep;
// Error: cannot implicitly convert expression dupDeep(a)
// of type dchar[] to string

Using `ElementType` means you get bitten by autodecoding. Since 
you know it's specifically an array, and not just any range, you 
can use `typeof(thing[0])` instead.

---

struct S { immutable int i; }
S a = S(123);
S b = a.dupDeep;
// Error: cannot modify struct instance `copy` of type `S`
// because it contains `const` or `immutable` members

I think the only way to make this work in the general case is 
with cooperation from the types, since the only way to initialize 
an `immutable` member is in a constructor. If you don't care 
about that, though, you can just slap an `if 
(isAssignable!(Unqual!T))` template constraint on the whole thing.