[ruby-core:67212] [ruby-trunk - Feature #10617] Change multiple assignment in conditional from parse error to warning

Issue #10617 has been updated by Recursive Madman.


I'm not sure if it's decent to push this - I understand 2.2.0 release is taking up a lot of time on the core devs side and holidays are also in progress in some parts of the world, so no hurry here.

Anyway, I feel I should ask:
* are there any known reasons not to accept this patch?
* does the issue need more clarification? #10450 has more information (I know that issue's discussion got a bit heated and confusing, which is why I opened this one to limit it to the actual change that was requested)

If there's anything more I should do to make this go through (maybe write a test or something?), I'm willing to do.

Ahoy.

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Feature #10617: Change multiple assignment in conditional from parse error to warning
https://bugs.ruby-lang.org/issues/10617#change-50689

* Author: Recursive Madman
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: 
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There is currently an inconsistency between regular and multiple assignment in conditionals.
Regular assignment causes a **warning**, multiple assignment causes a **parse error**.

The historical reason for this is that in 1.8 multiple assignment would always return an Array, but since 1.9 it returns whatever the RHS evaluates to.

**Examples:**

```ruby
a, b = nil  #=> nil
a, b = []   #=> []     (but a and b are both nil)
a, b = 1,2  #=> [1, 2]
```

Since multiple assignment behavior has changed, it makes sense to remove the (artificial) parse error for multiple assignments.

That makes it possible to test the return value of a method used for multiple assignment without having to use a temporary variable.

**Example:**

```ruby
# CURRENTLY WORKING CODE:
tmp = some_method_returning_array_or_nil
a, b = tmp
if tmp
  # method returned an array (possibly empty)
else
  # method returned nil.
end

# PROPOSED WORKING CODE:
if(a, b = some_method_returning_array_or_nil)
  # method returned an array (possibly empty)
else
  # method returned nil
end
```

(the parenthesis are needed due to LALR limitations, as discussed in #10450)

Attached is a patch that does the necessary change.


---Files--------------------------------
0001-turn-parse-error-on-multiple-assignment-into-warning.patch (752 Bytes)


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