Ruby 1.8 evaluates block argument out of order from other arguments and receiver
In Ruby 1.9 and all alternative impls I tested, block arguments are evaluated in the order they're encountered in a call. For example:
a.foo(b, c, &d)
The expected order would be left to right, a, then b, then c, then d. But Ruby 1.8 evaluates in the order d, a, b, c. In a case like the following (somewhat contrived) this would have odd side effects:
x = 0
(x += 1; a).foo(x += 1, x += 1, &(x += 1, d))
In 1.8, the two non-block arguments would be 3, 4, while on Ruby 1.9 and other impls it would be 2, 3 as you'd expect.