What's opIndexAssign supposed to return ?

[Petar]

On Tuesday, 25 February 2020 at 11:02:40 UTC, wjoe wrote:
> Lets say I've got 3 overloads of opIndexAssign:
>
> auto opIndexAssign(T t);
> auto opIndexAssign(T t, size_t i); and
> auto opIndexAssign(T t, size_t[2] i);
>
> I would assume to return what I would return with opIndex but 
> I'd rather not act upon assumptions.
> But if yes is it supposed to be the newly assigned values or 
> the pre-assignment ones ? By value or by reference ? And if 
> it's the new stuff can I just return t ?
>
> The language manual on operator overloading didn't answer that 
> question and neither did an internet search which didn't find 
> any useful information. Something unrelated and a heads up 
> about introducing opIndexAssign from 2004.

opIndexAssign is the operator used in the following code:

arr[1] = 8;

It returns the element at index 1 (so 8 in this case) by 
reference.
This allows you to do:

(arr[1] = 8)++;
assert(arr[1] == 9);

Whether or not you want to support this behavior in your custom 
data structure is up to you. It's perfectly valid to return the 
element by value or even return void.

Returning void from any custom assignment operator is always a 
safe choice. It's possible that some algorithms (e.g. in Phobos 
or third-party libraries) may need op*Assign to return something, 
but in that unlikely case you'll get a compile-time error, so it 
will be an easy fix.