[ruby-core:109818] [Ruby master Bug#18990] Pattern matching unexpectedly raises "duplicated key name" error

["henryanvyl (Henry Tseng)" <noreply@ruby-lang.org>]

Issue #18990 has been updated by henryanvyl (Henry Tseng).


Verifying that this occurs in ruby 3.1.0 but does not exist in 3.2.x

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Bug #18990: Pattern matching unexpectedly raises "duplicated key name" error
https://bugs.ruby-lang.org/issues/18990#change-99057

* Author: zeke (Zeke Gabrielse)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-darwin19]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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I've found that sometimes, when matching nested patterns, I get an unexpected "duplicate key name" error but there's no duplicate key in any individual pattern. The "duplicated" key is actually in another pattern that's in scope, which I found was odd. (And the fix is even more odd.)

This seems like unexpected behavior to me. There's an example of this behavior below. The patterns in #all? cause the error, even though they're separate lines.

```ruby
class User
  attr_reader :first_name,
              :last_name

  def initialize(first_name:, last_name:)
    @first_name, @last_name = first_name, last_name
  end

  def deconstruct_keys(keys)
    { first_name:, last_name: }
  end
end

users = [
  User.new(first_name: 'Jane', last_name: 'Doe'),
  User.new(first_name: 'Jane', last_name: 'Smith'),
]

# Fails
case users
in [{ first_name: 'John', ** }, *]
  users.all? { _1 in first_name: 'John' }
in [{ first_name: 'Jane', ** }, *]
  users.all? { _1 in first_name: 'Jane' } # => bug.rb:17: duplicated key name
end
```

Counterintuitively, wrapping the pattern in {} resolves the error.

```ruby
# Works
case users
in [{ first_name: 'John', ** }, *]
  users.all? { _1 in { first_name: 'John' } }
in [{ first_name: 'Jane', ** }, *]
  users.all? { _1 in { first_name: 'Jane' } }
end
```

Seems like a bug?



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