[ruby-core:102445] [Ruby master Bug#17619] if false foo=42; end creates a foo local variable set to nil

Issue #17619 has been updated by pkmuldoon (Phil Muldoon).


While the hoisting issue is meh to mostly okay, I guess, this block:

```
if false
   foo = 32
end
defined?(foo)
"local-variable"
```

is arguably an interpreter side effect that can cause problematic issues to developers. Especially if they use `defined?(foo)` as a conditional return value. If a condition in a control flow primitive equates to false, arguably, in the case above, that code is being executed, if only partially (the creation of a local variable `foo`), and causing side effects. If as a developer, I read that code above, I would expect that condition not to be executed, and `foo` remain undefined. Thanks for looking at this. Your attention to it valued!


----------------------------------------
Bug #17619: if false foo=42; end creates a foo local variable set to nil
https://bugs.ruby-lang.org/issues/17619#change-90328

* Author: pkmuldoon (Phil Muldoon)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Take this following code

```
[1] pry(main)> defined?(foo)
nil

[2] pry(main)> if false
[2] pry(main)*   foo = 42
[2] pry(main)* end  

[3] pry(main)> defined?(foo)
"local-variable"
```

The inner scope inherits the parent scope (ok) but also modifies the parent scope even if the child scope is never entered. A lesser effect of this:

```
[1] pry(main)> defined?(bar)
nil

[2] pry(main)> if false
[2] pry(main)*   bar = 99
[2] pry(main)* end  

[3] pry(main)> defined?(bar)
"local-variable"

[5] pry(main)> bar
99
```

That somewhat lesser affecting because I can just about accept a variable invading the parent scope, and existing after, as a hoisting event. But surely that should not be the case in the negative program-flow case?

The side effects of this are defined?(foo) can't be trusted anymore.

Apologies if this bug has been filed. I did search for it, but couldn't find anything quite matching it. Thanks!



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