[ruby-core:83595] [Ruby trunk Feature#10617] Change multiple assignment in conditional from parse error to warning

Issue #10617 has been updated by Eregon (Benoit Daloze).


This is now allowed, but produces a warning:

~~~
$ ruby -e 'if (a, b = 1, 2); puts "y"; end'
-e:1: warning: found = in conditional, should be ==
y

$ ruby -W0 -e 'if (a, b = 1, 2); puts "y"; end'
y
~~~

Is that expected?

It seems to only happen with literals though:

~~~
$ ruby -e 'if (a, b = 1, 2); puts "y"; end' 
-e:1: warning: found = in conditional, should be ==
y

$ ruby -e 'if (a, b = 1, 2.itself); puts "y"; end'
y
~~~

----------------------------------------
Feature #10617: Change multiple assignment in conditional from parse error to warning
https://bugs.ruby-lang.org/issues/10617#change-67622

* Author: recursive-madman (Recursive Madman)
* Status: Closed
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
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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