To summarize* and elaborate: Originally in Ruby, multiple assignment inside a conditional caused a parse error. Why? Because multiple assignment originally always returned an array:

  irb(main): a, b = nil
  => [nil]

In Ruby, non-empty arrays always evaluate to truthy. (The array above has one element, nil.) Hence, multiple assignment inside a conditional would have been meaningless. For this reason, Ruby threw a parse error.

As of Ruby 1.9, multiple assignment in a conditional now returns whatever the right-hand side of the assignment operator evaluates to. So, it makes sense to re-allow multiple assignment inside conditionals and remove the parse error.

Here are some examples illustrating the return value of multiple assignment. The first two are truthy while the third is falsey:

  irb(main): a, b = 8, 7
  => [8, 7]

  irb(main): a, b = 8
  => 8
  (a = 8, b = nil)

  irb(main): a, b = nil
  => nil
  (a = nil, b = nil)

- The above based on user 'bug hit' who attributes Yusuke Endoh, from https://bugs.ruby-lang.org/issues/10617