Issue #16166 has been updated by jeremyevans0 (Jeremy Evans).


matz (Yukihiro Matsumoto) wrote:
> I think the following code behavior is wrong:
> ```ruby
> p instance_exec(["a"]){|*foo, **bar| foo } #=> ["a"]
> ```
> It should return `[["a"]]`.

Here's a pull request for that: https://github.com/ruby/ruby/pull/2502

Note that it breaks some tests/specs.  I believe the reason methods with keywords were handled differently is because the last element of the argument could be used as keywords:

```
$ ruby -e "p proc{|*foo, **bar| [foo, bar]}.call([1, {a: 1}])"
-e:1: warning: The last argument is used as the keyword parameter
-e:1: warning: for `call' defined here
[[1], {:a=>1}]
```

As you can see, this now raises a warning in Ruby 2.7, and behavior will change in Ruby 3.0.  Do we want to make this change in 2.7, or do we want to wait for 3.0?


----------------------------------------
Feature #16166: Remove exceptional treatment of *foo when it is the sole block parameter
https://bugs.ruby-lang.org/issues/16166#change-81778

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
In the parameter signature of a code block for a method that is not involved in method definition or creation of lambda objects, two types of arguments `["a"]` and `"a"` are neutralized:

```ruby
instance_exec(["a"]){|foo, bar| foo} # => "a"
instance_exec("a"){|foo, bar| foo} # => "a"

instance_exec(["a"]){|*foo, **bar| foo} # => ["a"]
instance_exec("a"){|*foo, **bar| foo} # => ["a"]
```

This is the same behavior as with assignment constructions:

```ruby
foo, bar = ["a"]; foo # => "a"
foo, bar = "a"; foo # => "a"

*foo = ["a"]; foo # => ["a"]
*foo = "a"; foo # => ["a"]
```

And it contrasts with constructions involved in method definition or creation of lambda objects, where the distinction is preserved:

```ruby
lambda{|foo| foo}.call(["a"]) # => ["a"]
lambda{|foo| foo}.call("a") # => "a"

->(foo){foo}.call(["a"]) # => ["a"]
->(foo){foo}.call("a") # => "a"

lambda{|*foo| foo}.call(["a"]) # => [["a"]]
lambda{|*foo| foo}.call("a") # => ["a"]

->(*foo){foo}.call(["a"]) # => [["a"]]
->(*foo){foo}.call("a") # => ["a"]
```

However, when `*foo` is the sole parameter of a code block for a method that is not involved in method definition or creation of lambda objects, `["a"]` and `"a"` are not neutralized:

```ruby
instance_exec(["a"]){|*foo| foo} # => [["a"]]
instance_exec("a"){|*foo| foo} # => ["a"]
```

behaving in contrast to assignment constructions, and rather on a par with constructions involved in method definition or creation of lambda objects.

Particularly, existence or absence of another parameter `**bar` entirely changes what `foo` refers to:

```ruby
instance_exec(["a"]){|*foo| foo} # => [["a"]]
instance_exec(["a"]){|*foo, **bar| foo} # => ["a"]
```

I find this behavior inconsistent and confusing. I would like to request to remove this exceptional treatment of splatted parameter `*foo` when it is the sole parameter in a code block. I request this behavior:

```ruby
instance_exec(["a"]){|*foo| foo} # => ["a"]
```




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