gistfile1.rb

def foo(*a)
  # Isn't the * below redundant? Doesn't it cast a to an Array if it isn't one already?
  # But if you remove it the behaviour changes - how come?
  yield *a
end

puts foo(1, 2) { |a| a }

# MRI, JRuby, prints 1; Truffle prints 1 2

I think this will make the example a little more clear:

def foo(*a)
  yield a
  yield *a
end

foo(1, 2) { |a| p a }

In the first yield we are passing a single argument which is an array. Since the block accepts a single arg you get the whole array.

In the second yield you splatted the array back as two arguments to the yield. Since the block accepts a single arg you get the first element.

block argument passing is easily the most confusing part of Ruby...