Workaround JRuby incompatibilities

lib/rspec/support/method_signature_verifier.rb

@@ -33,6 +33,18 @@ def valid_non_kw_args?(positional_arg_count, optional_max_arg_count=positional_a
           optional_max_arg_count <= max_non_kw_args
       end
 
+      def classify_arity([email protected])
+        if arity < 0
+          # `~` inverts the one's complement and gives us the
+          # number of required args
+          @min_non_kw_args = ~arity
+          @max_non_kw_args = INFINITY
+        else
+          @min_non_kw_args = arity
+          @max_non_kw_args = arity
+        end
+      end
+
       if RubyFeatures.optional_and_splat_args_supported?
         def description
           @description ||= begin
@@ -137,36 +149,46 @@ def unlimited_args?
           false
         end
 
-        def classify_parameters
-          arity = @method.arity
-          if arity < 0
-            # `~` inverts the one's complement and gives us the
-            # number of required args
-            @min_non_kw_args = ~arity
-            @max_non_kw_args = INFINITY
-          else
-            @min_non_kw_args = arity
-            @max_non_kw_args = arity
-          end
-        end
+        alias_method :classify_parameters, :classify_arity
       end
 
       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
-    if RSpec::Support::Ruby.jruby? &&
-       RubyFeatures.optional_and_splat_args_supported? &&
-       Class.new { attr_writer :foo }.instance_method(:foo=).parameters == []
+    if RSpec::Support::Ruby.jruby?
+      # 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 RubyFeatures.optional_and_splat_args_supported? &&
+         Java::JavaLang::String.instance_method(:char_at).parameters == []
 
-      class MethodSignature < remove_const(:MethodSignature)
-      private
+        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
+          def classify_parameters
+            super
+            if (arity = @method.arity) != 0 && @method.parameters.empty?
+              classify_arity(arity)
+            end
+          end
+        end
+      end
+
+      # JRuby used to always report -1 arity for Java proxy methods
+      if Java::JavaLang::String.instance_method(:char_at).arity == -1
+        class MethodSignature < remove_const(:MethodSignature)
+        private
+
+          def classify_parameters
+            super
+            return unless @method.arity == -1 && @method.owner.respond_to?(:java_class)
+            java_instance_methods = @method.owner.java_class.java_instance_methods
+            compatible_overloads = java_instance_methods.select do |java_method|
+              @method == @method.owner.instance_method(java_method.name)
+            end
+            if compatible_overloads.size == 1
+              classify_arity(compatible_overloads.first.arity)
+            end
+          end
         end
       end
     end

spec/rspec/support/method_signature_verifier_spec.rb

@@ -719,6 +719,24 @@ 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
+
+            it 'fails validation against 0 arguments' do
+              expect(valid_non_kw_args?(0)).to eq false
+            end
+
+            it 'fails validation against 2 arguments' do
+              expect(valid_non_kw_args?(2)).to eq false
+            end
+          end
+        end
       end
 
       let(:fake_matcher) { Object.new }