[ruby-core:100039] [Ruby master Bug#17178] Procs with kw:def/**kw lose elements when called with a single Array

Issue #17178 has been updated by jeremyevans0 (Jeremy Evans).


Eregon (Benoit Daloze) wrote:
> ```
> $ ruby -ve '{a: 1}.each_pair { |a, kw: :def| p [a,kw] }'
> ruby 3.0.0dev (2020-09-18T09:12:58Z master e1cca08a6d) [x86_64-linux]
> [:a, :def] # bug, should be [[1, 2], :def]
> 
> $ ruby -ve '{a: 1}.each_pair { |a, **kw| p [a,kw] }' 
> ruby 3.0.0dev (2020-09-18T09:12:58Z master e1cca08a6d) [x86_64-linux]
> [:a, {}] # bug, should be [[1, 2], {}]
> ```
> So the value of the key-value pair is lost and not assigned to any argument.
> 
> This seems a general problem of Proc, not just #each_pair:
> ```
> $ ruby -e 'proc { |a, kw: :def| p [a,kw] }.call(1,2)'  
> [1, :def] # understandable
> $ ruby -e 'proc { |a, kw: :def| p [a,kw] }.call([1,2])'
> [1, :def] # bug, should be [[1, 2], :def]
> 
> 
> $ ruby -e 'proc { |a, **kw| p [a,kw] }.call(1,2)'     
> [1, {}] # understandable
> $ ruby -e 'proc { |a, **kw| p [a,kw] }.call([1,2])'
> [1, {}] # bug, should be [[1, 2], {}]
> ```

Are we sure this is a bug, and not just the result of a different behavior of `proc{|a|}` `proc{|a,|}`, with keywords being treated like the latter (autosplatting) as a comma is used?

Keywords in procs have always caused autosplatting of the proc.  With Ruby 3, we could change this if there were only a single argument and keywords, but it seems more like a new feature than a bug fix.


----------------------------------------
Bug #17178: Procs with kw:def/**kw lose elements when called with a single Array
https://bugs.ruby-lang.org/issues/17178#change-87589

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0dev (2020-09-18T09:12:58Z master e1cca08a6d) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
```
$ ruby -ve '{a: 1}.each_pair { |a, kw: :def| p [a,kw] }'
ruby 3.0.0dev (2020-09-18T09:12:58Z master e1cca08a6d) [x86_64-linux]
[:a, :def] # bug, should be [[1, 2], :def]

$ ruby -ve '{a: 1}.each_pair { |a, **kw| p [a,kw] }' 
ruby 3.0.0dev (2020-09-18T09:12:58Z master e1cca08a6d) [x86_64-linux]
[:a, {}] # bug, should be [[1, 2], {}]
```
So the value of the key-value pair is lost and not assigned to any argument.

This seems a general problem of Proc, not just #each_pair:
```
$ ruby -e 'proc { |a, kw: :def| p [a,kw] }.call(1,2)'  
[1, :def] # understandable
$ ruby -e 'proc { |a, kw: :def| p [a,kw] }.call([1,2])'
[1, :def] # bug, should be [[1, 2], :def]


$ ruby -e 'proc { |a, **kw| p [a,kw] }.call(1,2)'     
[1, {}] # understandable
$ ruby -e 'proc { |a, **kw| p [a,kw] }.call([1,2])'
[1, {}] # bug, should be [[1, 2], {}]
```



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