Issue #16432 has been updated by ko1 (Koichi Sasada).


* in short: 

```ruby
1.times{
  p _1 #=> 0
  eval("[:a, :b].each{p _1}")
  #=> 0
  #=> 0
}
```

mame: it can cause confusing bug.
ko1: we have two solutions:

1. separate numbered parameter scope between in/out eval.
  * Issue: we can't see `_1` variables from debugging reason.
    * allow `Binding#local_variable_get("_1")` for this purpose? How to get uplevel binding?

2. Same semantics without `eval`.
  * issue: we can't write numbered parameters in this kind of context.

```ruby
# 2.7.0 behavior
1.times{p _1
  eval('p _1')            #=> OK (0)
  eval('[:a].each{p _1}') #=> OK, but confusing (0)
}

# 1.
1.times{p _1
  eval('p _1')            #=> NameError
  eval('[:a].each{p _1}') #=> OK (:a)
}

# 2.
1.times{p _1
  eval('p _1')            #=> OK (0)
  eval('[:a].each{p _1}') #=> SyntaxError
}
```

```ruby
1.times{binding.irb}

# start IRB session
irb> p _1
#=>
# 2.7.0 NameError
# 1. NameError
# 2. NameError

1.times{binding.irb;_1}
# start IRB session
irb> p _1
#=>
# 2.7.0 OK (0)
# 1. NameError
# 2. OK (0)

irb> [:a].each{p _1}
#=>
# 2.7.0 OK, but confusing (0)
# 1. OK (:a)
# 2. SyntaxError
```

matz: vote for 1

Conclusion:

* Solution 1. Nobu will fix parse.y
* hopefully backport to 2.7.1 (March 2020)


----------------------------------------
Feature #16432: Using `_1` inside `binding.irb` will cause unintended behavior
https://bugs.ruby-lang.org/issues/16432#change-83909

* Author: osyo (manga osyo)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
## Summary

Calling `binding.irb` in a block that uses `_1` and using `_1` in `irb` will cause unintended behavior.


## Steps to reproduce

1. Use `_1` in block
2. Use `binding.irb` in block
3. Use block with `_1` in `binding.irb`

```ruby
# test.rb
proc {
  binding.irb
  _1
}.call 42
```

```ruby
# binding.irb
irb> (1..10).map { _1 + _1 }
```


## Expected behavior

```ruby
irb> _1
 => 42
irb> (1..10).map { _1 + _1 }
 = > [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
```


## Actual behavior

* Refers to the `_1` of the block that called` binding.irb`

```ruby
irb> _1
 => 42
irb> (1..10).map { _1 + _1 }
 = > [84, 84, 84, 84, 84, 84, 84, 84, 84, 84]
```

I think this is an unintended behavior for the user.






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