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Bug #17596

closed

Calling parameters on `new` method results in :rest and -1 arity

Added by baweaver (Brandon Weaver) 12 months ago. Updated 12 months ago.

Status:
Rejected
Priority:
Normal
Assignee:
-
Target version:
-
[ruby-core:102318]

Description

Consider the following class:

class Testing
  def initialize(a, b, c)
    # ...
  end
end

...and the following call:

m = Testing.method(:new)
p arity: m.arity, params: m.parameters

Expected

{ arity: 3, params: [[:req, :a], [:req, :b], [:req, :c]] }

Actual

{ arity: -1, params: [[:rest]] }

This is confusing to me as I would expect to be able to see the parameters of the new method like this. The same applies for `initialize.

I was experimenting with pattern matching interfaces and dynamically defining class constructor definitions as such:

def deconstruct
  method(:new).parameters.map { public_send(_1.last) }
end

While this case is not 100% safe for all initializers I had expected it to work for testing and was surprised when it did not.

I've tested and confirmed this behavior back to 2.5 and tested no further back

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