[ruby-core:114336] [Ruby master Bug#19795] attr_accessor leading to nil values for re-assignment

Issue #19795 has been updated by francktrouillez (Franck Trouillez).


I got it in the end thanks to a peer. I misunderstood wrongly how this was working:

Running the following snippet helped me understand:
```ruby
# attr_accessor_nil.rb

class A
  attr_accessor :a

  def initialize
    @a = 0
  end

  def my_method
    puts "a is '#{a.inspect}' of class '#{a.class}'"
    b = a + 1
    a = b
    puts "b is '#{b.inspect}' of class '#{b.class}'"
    puts "a is '#{a.inspect}' of class '#{a.class}'"
  end

  def my_method_2
    puts "a is '#{a.inspect}' of class '#{a.class}'"
  end
end

instance = A.new
instance.my_method
instance.my_method_2

# output:
#
# a is '0' of class 'Integer'
# b is '1' of class 'Integer'
# a is '1' of class 'Integer'
# a is '0' of class 'Integer'
```

So, we cannot modify the instance variable through the accessor directly. If it was, the last `puts` should show `1`

Thanks for your help!

----------------------------------------
Bug #19795: attr_accessor leading to nil values for re-assignment
https://bugs.ruby-lang.org/issues/19795#change-104041

* Author: francktrouillez (Franck Trouillez)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
### Steps to reproduce:
- Create a class with an `attr_accessor` for an instance variable
- Create an instance method that reassign this variable using the current value stored in the variable
- Show that the variable is set to nil during the evaluation

Code snippet:
``` ruby
# attr_accessor_nil.rb

class A
  attr_accessor :a

  def initialize
    @a = 0
  end

  def my_method
    puts "a is '#{a.inspect}' of class '#{a.class}'"
    a += 1
  end
end

instance = A.new
instance.my_method

# output:
#
# a is '0' of class 'Integer'
# attr_accessor_nil.rb:12:in `my_method': undefined method `+' for nil:NilClass (NoMethodError)
#
#     a += 1
#       ^
# 	from attr_accessor_nil.rb:17:in `<main>'


```

### Expected behavior
`a += 1` should lead to `a` being equal to `1` at the end of the assignment, because `a` was storing `0` previously, as shown by the `puts`. Am I being wrong expecting this result?

### Actual behavior
`a += 1` raises an error about `a` being `nil` in the evaluation.

### Further investigation
I checked if it was coming from the "instance variable", or about the "attr_accessor" by running the following snippet:

Code snippet:
```ruby
# attr_accessor_nil.rb

class A
  attr_accessor :a

  def initialize
    @a = 0
  end

  def my_method
    puts "a is '#{a.inspect}' of class '#{a.class}'"
    @a += 1 # use the instance variable directly, instead of the accessor
  end
end

instance = A.new
instance.my_method

# output:
#
# a is '0' of class 'Integer'

```

This snippet runs just fine, and no error is raised.


### System configuration
Ruby version : 3.2.2




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