[ruby-core:99077] [Ruby master Bug#17015] RubyVM::AbstractSyntaxTree.parse has the same result on `proc { |a| }` and `proc { |a, | }`

Issue #17015 has been reported by osyo (manga osyo).

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Bug #17015: RubyVM::AbstractSyntaxTree.parse has the same result on `proc { |a| }` and `proc { |a,| }`
https://bugs.ruby-lang.org/issues/17015

* Author: osyo (manga osyo)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.8.0dev (2020-07-05T16:15:47Z master efe851a0df) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
## Summary

`RubyVM::AbstractSyntaxTree.parse("proc { |a| }")` and `RubyVM::AbstractSyntaxTree.parse("proc { |a,| }")` have the same result.
Since `proc { |a| }` and `proc { |a,| }` have different meanings, I think we need a way to distinguish them.

```ruby
node1 = RubyVM::AbstractSyntaxTree.parse("proc { |a| }")
body1 = node1.children.last
pp body1
# => (ITER@1:0-1:12 (FCALL@1:0-1:4 :proc nil)
#       (SCOPE@1:5-1:12
#        tbl: [:a]
#        args:
#          (ARGS@1:8-1:9
#           pre_num: 1
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: nil
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (BEGIN@1:10-1:10 nil)))


# returns the same result as proc { |a| }
node2 = RubyVM::AbstractSyntaxTree.parse("proc { |a,| }")
body2 = node2.children.last
pp body2
# => (ITER@1:0-1:13 (FCALL@1:0-1:4 :proc nil)
#       (SCOPE@1:5-1:13
#        tbl: [:a]
#        args:
#          (ARGS@1:8-1:10
#           pre_num: 1
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: nil
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (BEGIN@1:11-1:11 nil)))
```

## Proposal

In case of `proc { |a,| }`, `rest: :NODE_SPECIAL_EXCESSIVE_COMMA` is added.

```ruby
# no change
node1 = RubyVM::AbstractSyntaxTree.parse("proc { |a| }")
body1 = node1.children.last
pp body1
# => (ITER@1:0-1:12 (FCALL@1:0-1:4 :proc nil)
#       (SCOPE@1:5-1:12
#        tbl: [:a]
#        args:
#          (ARGS@1:8-1:9
#           pre_num: 1
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: nil
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (BEGIN@1:10-1:10 nil)))


# rest: :NODE_SPECIAL_EXCESSIVE_COMMA is added
node2 = RubyVM::AbstractSyntaxTree.parse("proc { |a,| }")
body2 = node2.children.last
pp body2
# => (ITER@1:0-1:13 (FCALL@1:0-1:4 :proc nil)
#       (SCOPE@1:5-1:13
#        tbl: [:a]
#        args:
#          (ARGS@1:8-1:10
#           pre_num: 1
#           pre_init: nil
#           opt: nil
#           first_post: nil
#           post_num: 0
#           post_init: nil
#           rest: :NODE_SPECIAL_EXCESSIVE_COMMA
#           kw: nil
#           kwrest: nil
#           block: nil)
#        body: (BEGIN@1:11-1:11 nil)))
```

This correspondence is similar to `:NODE_SPECIAL_NO_NAME_REST` for `MASGN`.

```ruby
node = RubyVM::AbstractSyntaxTree.parse("*a = foo")
body = node.children.last
pp body
# => (MASGN@1:0-1:8 (VCALL@1:5-1:8 :foo) nil (LASGN@1:1-1:2 :a nil))

# :NODE_SPECIAL_NO_NAME_REST is added if the variable name is not defined
node = RubyVM::AbstractSyntaxTree.parse("* = foo")
body = node.children.last
pp body
# => (MASGN@1:0-1:7 (VCALL@1:4-1:7 :foo) nil :NODE_SPECIAL_NO_NAME_REST)
```

## NOTE

`NODE_SPECIAL_EXCESSIVE_COMMA` was implemented in [0872ea5](https://github.com/ruby/ruby/commit/0872ea53303499caf3584e40f2a5438e86eb4fed) and renamed in で実装され、[f258137](https://github.com/ruby/ruby/commit/f258137083051a7fc2412c62e3fb239f93d1fdf8).
So backport to 2.6 can be difficult.

## Versions

* `ruby 2.6.6p146 (2020-03-31 revision 67876) [x86_64-linux]`
* `ruby 2.7.1p83 (2020-03-31 revision a0c7c23c9c) [x86_64-linux]`
* `ruby 2.8.0dev (2020-07-05T16:15:47Z master efe851a0df) [x86_64-linux]`

## Patch

* PR:https://github.com/ruby/ruby/pull/3298




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