When a call is made with paren-less arguments and a do...end block, the shorthand Proc#call syntaxes .() and ::() do not parse.
o = Object.new
def o.m(*arg); proc {|a| arg.join + a }; end
a = o.m "foo", "bar" do end.("buz")
b = o.m "foo", "bar" do end::("buz")
This causes a failure in CRuby's test_parse.rb @ test_call_command.