From: ruby-core@...
Date: 2019-05-27T16:37:02+00:00
Subject: [ruby-core:92865] [Ruby trunk Bug#15880] Wrong precedence of the if modifier in pattern matching

Issue #15880 has been updated by marcandre (Marc-Andre Lafortune).

Status changed from Open to Rejected

This is necessary so the `if` can depend on the variables of the matching, e.g. `in [Integer => x] if x.odd?`


----------------------------------------
Bug #15880: Wrong precedence of the if modifier in pattern matching
https://bugs.ruby-lang.org/issues/15880#change-78244

* Author: ibylich (Ilya Bylich)
* Status: Rejected
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-05-20 trunk ab0f2deab1) [x86_64-darwin18]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
When "If" is used as an "If modifier" it runs before the expression that it wraps:
``` ruby
=> puts 1 if (puts 2; true)
2
1
```

However, when it's used in the pattern matching destructuring runs first:
``` ruby
class A
  def deconstruct
    puts 'deconstruct called'
    [1]
  end
end

p case A.new
in A[1] if (puts 'if check'; true)
  'yes'
else
  'no'
end

# prints
# deconstruct called
# if check
# "yes"
```

I personally think that it's very confusing as it doesn't reflect the code. I assumed it to not run destructuring if the check returns `false` (especially because destructuring is allowed to have side-effects)



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