[ruby-core:95801] [Ruby master Bug#16343] Inconsistent behavior of 'circular argument reference' error

Issue #16343 has been updated by jeremyevans0 (Jeremy Evans).


On Ruby 2.2-2.6, there is a warning instead of an error in the first case, but I think that is invalid.  The reason for the warning/error is to complain about this code:

```ruby
def a(b=b)
  b
end
```

Here, `a` would return `nil`, because of how the default argument values are handled.  The reason the warning was added is because in Ruby 2.1 and below, calling `a` with no arguments would call the `b` method to get the value for the `b` argument.

`m { |foo = proc { || foo }|  }` is not a case I think we should warn about, since it doesn't have the same issue.  `m { |foo = proc { || foo }.call|  }` could have the same issue, but it seems unlikely to occur in real world code.

----------------------------------------
Bug #16343: Inconsistent behavior of 'circular argument reference' error
https://bugs.ruby-lang.org/issues/16343#change-82622

* Author: ibylich (Ilya Bylich)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-11-04T16:32:26Z trunk a087e027bf) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
The following code gives a syntax error:
``` ruby
m { |foo = proc { || foo }|  }
# => SyntaxError (-e:14: circular argument reference - foo)
```

But this code is valid:
``` ruby
m { |foo = proc { |bar| foo }|  }
```

Is it intentional? `foo` becomes a proc that returns itself, so there must be a circular dependency




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