[ruby-core:80995] [Ruby trunk Bug#13543] local variable declaration

Issue #13543 has been reported by eiko (eiko kokuma).

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Bug #13543: local variable declaration
https://bugs.ruby-lang.org/issues/13543

* Author: eiko (eiko kokuma)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
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The following code snippet generates a strange error:

~~~ ruby
def foo
  @foo ||= "foo"
end

foo = foo.size

#=> undefined method 'size' for nil:NilClass (NoMethodError)
~~~

I expect the code to create a local variable `foo` which is assigned the value `3`. I expect this because in assignment, the right size of `=` is always resolved before being bound to the left side of `=`. In my mind, `foo` should refer to the `foo()` method right up to the point that a value is assigned to the local variable which shadows it. It is confusing and unexpected to have a period of limbo in which `foo()` is overshadowed by an unassigned local variable mid-declaration.

In other languages, declaration and initial assignments are atomic, so this code would throw errors:

In python:

~~~ python
foo = foo
# NameError: name 'foo' is not defined
~~~

In rust:
~~~ c
let foo = foo
// error: cannot find value 'foo' in this scope.
~~~

But in ruby, a variable can be instantly declared and assigned to itself:
~~~ ruby
foo = foo
#=> nil
~~~

This goes against the common motifs of programming in ruby and other languages and can lead to the confusing error mentioned above. Is there a reason local variables should be declared prior to resolving the assignment expression in ruby? I could find no documented reason for this design choice, leading me to believe it is a bug.



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