There appears to be no benefit in actually looking into all branches and checking if all branches reassign since even then the outer assignment defines the variable.

I don't quite get this part, as by looking at them you can actually determine whether some assignment is void or not.

There appears to be no benefit in actually looking into all branches and checking if all branches reassign since even then the outer assignment defines the variable.

I don't quite get this part, as by looking at them you can actually determine whether some assignment is void or not.

Can you create an example? What I was trying to highlight is situations like this:

changed = false

if Random.rand > 1
  changed = true
else 
  changed = false
end

[].each do
  changed = true
end

puts changed

Even considering that any branch will assign to changed the assignment above is not useless since it creates the variable.
Out of the top of my head no examples come to mind that it matters to check all branches.

Even considering that any branch will assign to changed the assignment above is not useless since it creates the variable.

So, your point applies only if there's a usage outside the branches, right? (in your case the puts)

Even considering that any branch will assign to changed the assignment above is not useless since it creates the variable.

So, your point applies only if there's a usage outside the branches, right? (in your case the puts)

I'd say so. And testing with, for instance:

def some_method
  changed = false
  ^^^^^^^ Useless assignment to variable - `changed`.

  if Random.rand > 1
    changed = true
    ^^^^^^^ Useless assignment to variable - `changed`.
  else
    changed = false
    ^^^^^^^ Useless assignment to variable - `changed`.
  end
end

works as expected. However, one important detail, in the following scenario:

def some_method
  changed = false

  if Random.rand > 1
    changed = true
  end

  [].each do
    changed = true
  end
end

the variable is not unused since it is captured by a block. This is respectful of the comments under RuboCop::Cop::VariableForce::Variable::used? that quote the following:

    # This is a convenient way to check whether the variable is used
   # in its entire variable lifetime.
   # For more precise usage check, refer Assignment#used?.
   #
   # Once the variable is captured by a block, we have no idea
   # when, where, and how many times the block would be invoked.
   # This means we cannot track the usage of the variable.
  # So we consider it's used to suppress false positive offenses.

@bbatsov