Environment

• jruby 9.1.6.0-SNAPSHOT (2.3.1) 2016-11-08 cb5a7be Java HotSpot(TM) 64-Bit Server VM 25.91-b14 on 1.8.0_91-b14 +jit [linux-x86_64]
• Linux noweto 4.4.0-25-generic LocalContextScope documentation improvement #44-Ubuntu SMP Fri Jun 10 18:19:48 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

Expected Behavior

The following worked at least until JRuby 9.0.0.0 and works in MRI:

def modify(x)
  x << 'a'
end

modify(u = '')     # => "a"
u                  # => "a"
x = {}             # => {}
modify(x[5] = '')  # => "a"
x                  # => {5=>"a"}

Actual Behavior

At least from JRuby 9.1.5.0 onwards including the above listed JRuby 9.1.6.0 snapshot the following happens:

def modify(x)
  x << 'a'
end

modify(u = '')     # => "a"
u                  # => "a"
x = {}             # => {}
modify(x[5] = '')  # => "a"
x                  # => {5=>""}

The difference is in the last line where the string assigned to x[5] isn't modified anymore, leading to erroneous behaviour.