While discussing a weird issue around treating lambdas as blocks in MRI (https://bugs.ruby-lang.org/issues/9605) @nobu found this odd behavior in jruby.

In jruby lambdas converted to blocks behave differently depending on whether they were created with the lambda or -> syntax.

1. As expected lambdas enforce arity with both syntaxes when called.

   $ jruby -v -e 'def test(l); l.call(1); end; test(lambda{p :ng})'
   jruby 1.7.10 (1.9.3p392) 2014-01-09 c4ecd6b on Java HotSpot(TM) 64-Bit Server VM 1.7.051-b13 [darwin-x8664]
   ArgumentError: wrong number of arguments (1 for 0)
   call at org/jruby/RubyProc.java:267
   test at -e:1
   (root) at -e:1
   
   $ jruby -v -e 'def test(l); l.call(1); end; test(->(){p :ng})' 
   jruby 1.7.10 (1.9.3p392) 2014-01-09 c4ecd6b on Java HotSpot(TM) 64-Bit Server VM 1.7.051-b13 [darwin-x8664]
   ArgumentError: wrong number of arguments (1 for 0)
   call at org/jruby/RubyProc.java:267
   test at -e:1
   (root) at -e:1
   

2. Surprisingly, when converted to a block lambdas behave differently depending on which syntax they were created with. The arity check is gone from those created with lambda

   $ jruby -v -e 'def test; yield 1; end; test(&->(){p :ng})' 
   jruby 1.7.10 (1.9.3p392) 2014-01-09 c4ecd6b on Java HotSpot(TM) 64-Bit Server VM 1.7.051-b13 [darwin-x8664]
   ArgumentError: wrong number of arguments (1 for 0)
   test at -e:1
   (root) at -e:1
   
   $ jruby -v -e 'def test; yield 1; end; test(&lambda{p :ng})' 
   jruby 1.7.10 (1.9.3p392) 2014-01-09 c4ecd6b on Java HotSpot(TM) 64-Bit Server VM 1.7.051-b13 [darwin-x8664]
   :ng