Workaround buggy Method#parameters in JRuby

JRuby apparently only supports #arity (and not #parameters) for Java proxy methods (see jruby/jruby#2817), but verifying doubles in RSpec use #parameters to infer required method parameters, and thus doesn't work for Java classes.

Given that this issue affects every release of JRuby so far with support for #parameters, it seemed reasonable to replace the previous workaround that only affected JRuby 1.7.0 through 1.7.19.

Author: grddev

lib/rspec/support/method_signature_verifier.rb

@@ -154,19 +154,26 @@ def classify_parameters
       INFINITY = 1 / 0.0
     end
 
-    # Some versions of JRuby have a nasty bug we have to work around :(.
-    # https://github.com/jruby/jruby/issues/2816
+    # JRuby has only partial support for UnboundMethod#parameters, so we fall back on using #arity
+    # https://github.com/jruby/jruby/issues/2816 and https://github.com/jruby/jruby/issues/2817
     if RSpec::Support::Ruby.jruby? &&
        RubyFeatures.optional_and_splat_args_supported? &&
-       Class.new { attr_writer :foo }.instance_method(:foo=).parameters == []
+       Java::JavaLang::String.instance_method(:char_at).parameters == []
 
       class MethodSignature < remove_const(:MethodSignature)
       private
 
         def classify_parameters
           super
-          return unless @method.parameters == [] && @method.arity == 1
-          @max_non_kw_args = @min_non_kw_args = 1
+          if (arity = @method.arity) != 0 && @method.parameters.empty?
+            if arity < 0
+              @min_non_kw_args = ~arity
+              @max_non_kw_args = INFINITY
+            else
+              @min_non_kw_args = arity
+              @max_non_kw_args = arity
+            end
+          end
         end
       end
     end

spec/rspec/support/method_signature_verifier_spec.rb

@@ -719,6 +719,16 @@ def arity_block(_, &block); end
             expect(valid_non_kw_args?(2)).to eq false
           end
         end
+
+        if Ruby.jruby?
+          describe 'a single-argument Java method' do
+            let(:test_method) { Java::JavaLang::String.instance_method(:char_at) }
+
+            it 'validates against a single argument' do
+              expect(valid_non_kw_args?(1)).to eq true
+            end
+          end
+        end
       end
 
       let(:fake_matcher) { Object.new }