In https://bugs.ruby-lang.org/issues/12860 I argue that MRI's
execution order here is incorrect. The splatting of the 'c' args
should happen before the shift, but it happens after. On JRuby, it
behaves the way you would expect, leading to the 'c' args splat
still containing the block and producing an error like "cannot
convert proc to symbol" when the send attempts to coerce
it.

This patch makes the unpacking order explicit with a multi-assign,
which behaves properly on all implementations I tested.