[ruby-core:95996] [Ruby master Feature#16166] Remove exceptional treatment of *foo when it is the sole block parameter

Issue #16166 has been updated by Eregon (Benoit Daloze).


@sawa wrote this in the dev-meeting ticket:
> Unintended arity. This must be fixed in an earlier stage before Ruby 3.

I think matz conclusion is all behavior shown in this bug so far is intended, except for `*foo, **bar`.

@Dan0042
```
proc{ |a,b| [a,b] }.call(1,2)   #=> [1, 2]
proc{ |*ab| ab    }.call(1,2)   #=> [1, 2]
proc{ |a,b| [a,b] }.call([1,2]) #=> [1, 2]
proc{ |*ab| ab    }.call([1,2]) #=> [[1, 2]]
```
That's just how Proc works, multiple parameters will splat an Array if a single Array argument is given.

I think long-term we might want to use lambda semantics by default for blocks, which doesn't have that splatting magic.

----------------------------------------
Feature #16166: Remove exceptional treatment of *foo when it is the sole block parameter
https://bugs.ruby-lang.org/issues/16166#change-82832

* 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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