This seems somewhat related to #2512, and may even be caused by the same problem, but it causes a a different problem, so I thought I'd report it separately.

Offending code looks like this:

def call_block(&block)
  block.call("foo", bar: "quox")
end

call_block { |foo, bar:| puts bar }

On MRI this correctly produces the string "quox", but on JRuby (master), this produces an error like this:

ArgumentError: missing keyword: bar
          call at org/jruby/RubyProc.java:289
  call_block_1 at test.rb:2
    __script__ at test.rb:5

Interestingly, omitting the first parameter to the block (foo in the example above) makes the code run without problem. The following correctly prints "quox" on JRuby master:

def call_block(&block)
  block.call(bar: "quox")
end

call_block { |bar:| puts bar }