Bug 14984 – Make it illegal (or at least a warning) to modify the iteration variable in foreach

Modifying the index variable in `foreach` is almost always wrong, for example: https://github.com/D-Programming-Language/dmd/pull/4991 Thus, it should be deprecated and later forbidden. If someone really wants to, they can use a normal `for` loop.

Currently it's intentional. When you declare a index variable in foreach statement, it works like an auto variable scoped in the foreach body. If you need non-mutable index variable, you can write code like: foreach (const i, v; arr) { ... } foreach (const i; 0..len) { ... }

@Kenji Hara: Yes, I know that. My point is that it's too easy to forget to add `const`.

(In reply to Marc Schütz from comment #3) > @Kenji Hara: > > Yes, I know that. My point is that it's too easy to forget to add `const`. It's not a special `foreach` issue. It's a mistake to modify a variable in most cases as often it is just a constant view of data so if one doesn't follow "everything must be marked `const` except it is definitely mutable" it's his coding style problem. Unfortunately D doesn't solve this issue as it doesn't assume `const` by default. I hope the next D version will do it.

Right now, if we modify the element of a foreach range, there is no problem because we are operating on a copy of the original element. If we need to modify the actual element, then we need to add ref. That stands true for the iteration variable also. I don't see why we would deprecate any of this behavior because it offers maximum flexibility.