[ruby-core:109730] [Ruby master Feature#18408] Allow pattern match to set instance variables

Issue #18408 has been updated by Eregon (Benoit Daloze).


Right, and I agree with matz that modifying ivars as the match is evaluated is problematic, I would even say it's unsound for pattern matching to have such side effects.
Testing a pattern should as much as feasible not have side effects.
(I think on local variables it's OK because of the very small scope they can affect but it can also be fixed there if we want)
For example:
```ruby
case [1, 2]
in [@v, 42] if v.even? then 42
in [a, b] then a + b
end
```
would have the side effect of assigning `@v` even though that pattern does not match.
That is very wrong and a sure way to break the object state, consistency and invariants in no time.

----------------------------------------
Feature #18408: Allow pattern match to set instance variables
https://bugs.ruby-lang.org/issues/18408#change-98965

* Author: Dan0042 (Daniel DeLorme)
* Status: Rejected
* Priority: Normal
* Assignee: ktsj (Kazuki Tsujimoto)
----------------------------------------
I expected this to work:

```ruby
42 => @v
```

But instead it raises "syntax error, unexpected instance variable"

Is this intentional?



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