FWIW, here's output from and patch to enable "rich" parameter
information on JRuby. We do not provide the argument names because
JRuby often implements multiple-arity methods with multiple native
code bodies (so there's potentially different argument names for each
arity). I would not expect to require parameter names for native
methods, since there will be many ways to implement native methods and
some may be incompatible with a single variable name for each
position.

The variable names are not very useful anyway...I think most people
will be interested in the parameter types.

system ~/projects/jruby $ jruby -e "p String.instance_method(:gsub).parameters"
[[:req], [:opt]]

system ~/projects/jruby $ jruby -e "p Array.instance_method(:[]=).parameters"
[[:req], [:req], [:opt]]

system ~/projects/jruby $ git diff
diff --git a/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
b/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
index dc45ae9..014f09a 100644
--- a/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
+++ b/core/src/main/java/org/jruby/internal/runtime/methods/InvocationMethodFactory.java
@@ -602,7 +602,7 @@ public class InvocationMethodFactory extends
MethodFactory implements Opcodes {
         private boolean block;
         private String parameterDesc;

-        private static final boolean RICH_NATIVE_METHOD_PARAMETERS = false;
+        private static final boolean RICH_NATIVE_METHOD_PARAMETERS = true;

         public DescriptorInfo(List<JavaMethodDescriptor> descs) {
             min = Integer.MAX_VALUE;

On Wed, Jul 10, 2013 at 1:32 PM, Charles Oliver Nutter
<headius / headius.com> wrote:
> On Wed, Jul 10, 2013 at 11:16 AM, Yorick Peterse
> <yorickpeterse / gmail.com> wrote:
>> The problem, at least with MRI, is that the moment you do something
>> similar with methods that are written in C all meaningful information is
>> lost:
>>
>>     String.instance_method(:gsub).parameters # => [[:rest]]
>>
>> This is false since gsub has at least 1 required argument. This happens
>> with a lot of methods (if not all) in MRI that are implemented in C.
>> Jruby is also affected by this (at least with the above example).
>> Rubinius is thus far the only implementation that gets this right that I
>> know of.
>
> Yes, that is the reason I filed this feature. :-)
>
> Rubinius diverges from everyone else here and presents its own
> argument list for #parameters rather than presenting the same
> information as MRI. I would like to see #parameters reflect meaningful
> information for even native methods, but the JRuby policy is to not
> unilaterally make such decisions.
>
> We could (and at one point, did) present the same data as Rubinius,
> but up to now we have chosen to match MRI.
>
> - Charlie
>