Why is opIndexAssign replaced by opSlice here?

[Elmar]

On Monday, 18 October 2021 at 03:42:35 UTC, Paul Backus wrote:
> What happens here is, the compiler first tries the D2-style 
> rewrite:
>
> ```d
> s.opIndexAssign(arr[1..4], s.opSlice!0(0, 3))
> ```
>
> However, that rewrite fails to compile, because your `opSlice` 
> does not take a template argument specifying the dimension 
> along which to slice, as specified in the language spec's 
> section on ["Slice Operator Overloading".][1]
>
> Since the above rewrite fails to compile, it falls back to 
> rewriting the expression using D1-style operator overloads:
>
>> For backward compatibility, `a[]` and `a[i..j]` can also be 
>> overloaded by implementing `opSlice()` with no arguments and 
>> `opSlice(i, j)` with two arguments, respectively. This only 
>> applies for one-dimensional slicing, and dates from when D did 
>> not have full support for multidimensional arrays. This usage 
>> of opSlice is discouraged.
>
> ...which results in the following rewritten statement:
>
> ```d
> s.opSlice(0, 3) = arr[1..4];
> ```
>
> My guess is that you got into this situation by trying to 
> follow the example in the spec's section on ["Slice Assignment 
> Operator Overloading"][2]. Unfortunately, that example is 
> incorrect.
>
> Here is a fixed version of your example on run.dlang.io: 
> https://run.dlang.io/is/dtfT5y
>
> [1]: https://dlang.org/spec/operatoroverloading.html#slice
> [2]: 
> https://dlang.org/spec/operatoroverloading.html#slice_assignment_operator

Woow! You fixed the problem, it now works for me.

I wish the compiler would have been able to make me understand 
the problem. It was no help because it only tells the final 
erroneous consequence, that the return value of `opSlice` cannot 
be assigned ("is no lvalue", beginner-unfriendly language). It 
doesn't even mention `opSlice` or the fallback (reason) with no 
warning.