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.
