Our core methods report the incorrect parameters when wrapped in a Method or UnboundMethod.

[] ~/projects/jruby/hutch $ rvm ruby-2.5.3 do ruby -e 'class IO; m = instance_method(:write); p m.parameters; define_method(:write, m); end; $stderr.puts IO.instance_method(:write).parameters'
[[:rest]]
rest

[] ~/projects/jruby/hutch $ jruby -e 'class IO; m = instance_method(:write); p m.parameters; define_method(:write, m); end; $stderr.puts IO.instance_method(:write).parameters'
[[:req]]
req

As shown here, for an unlimited-arity method like IO#write we report :req which means single required argument. However the actual #arity method reports -1, indicating varargs.

This is the cause of issues reported by @olleolleolle on our chat. In his case, this bug caused RSpec's signature-verification (which appears to be used by mocking/proxying method logic) to attempt to verify that the the mock method has the same arity as the original, using parameters to build up a parameter description. This description said that IO#write should only take one argument, so a two-argument call fails to verify and it blows up within RSpec internals.

#<ArgumentError: Wrong number of arguments. Expected 1, got 2.>
#<Errno::ENOENT: No such file or directory - /path/to/nonexistant/file>
["/Users/olle/.rvm/gems/jruby-9.2.6.0/gems/rspec-mocks-3.8.0/lib/rspec/mocks/verifying_proxy.rb:167:in `block in validate_arguments!'",
 "/Users/olle/.rvm/gems/jruby-9.2.6.0/gems/rspec-mocks-3.8.0/lib/rspec/mocks/verifying_proxy.rb:191:in `with_signature'",
 "/Users/olle/.rvm/gems/jruby-9.2.6.0/gems/rspec-mocks-3.8.0/lib/rspec/mocks/verifying_proxy.rb:165:in `validate_arguments!'",
 "/Users/olle/.rvm/gems/jruby-9.2.6.0/gems/rspec-mocks-3.8.0/lib/rspec/mocks/verifying_proxy.rb:160:in `proxy_method_invoked'",
 "/Users/olle/.rvm/gems/jruby-9.2.6.0/gems/rspec-mocks-3.8.0/lib/rspec/mocks/method_double.rb:64:in `block in define_proxy_method'",
 "org/jruby/RubyIO.java:2638:in `write'",

Although this looks like one of our ArgumentError message, it actually comes from RSpec's signature verifier.

The root cause was this line in DescriptorInfo:

This line assumes that if the minimum number of arguments matches the maximum number of arguments, it must be a fixed number of arguments, in which case it should just be a single :req symbol. However in this logic, if the maximum and minimum are both -1, that means it's a rest arg.

I have a fix in process locally. I will also try to start reducing the duplication between that class and ArgumentType.