fix warning related to keywoard argument (#45)

* add workaround to avoid warnings related to keyword arguments (refs: #44)

lib/docile/fallback_context_proxy.rb

@@ -82,17 +82,25 @@ def instance_variables
 
     # Proxy all methods, excluding {NON_PROXIED_METHODS}, first to `receiver`
     # and then to `fallback` if not found.
-    def method_missing(method, *args, &block)
-      if @__receiver__.respond_to?(method.to_sym)
-        @__receiver__.__send__(method.to_sym, *args, &block)
+    args_string =
+      if RUBY_VERSION >= "2.7.0"
+        "*args, **kwargs, &block"
       else
-        begin
-          @__fallback__.__send__(method.to_sym, *args, &block)
-        rescue NoMethodError => e
-          e.extend(BacktraceFilter)
-          raise e
+        "*args, &block"
+      end
+    class_eval(<<-METHOD)
+      def method_missing(method, #{args_string})
+        if @__receiver__.respond_to?(method.to_sym)
+          @__receiver__.__send__(method.to_sym, #{args_string})
+        else
+          begin
+            @__fallback__.__send__(method.to_sym, #{args_string})
+          rescue NoMethodError => e
+            e.extend(BacktraceFilter)
+            raise e
+          end
         end
       end
-    end
+    METHOD
   end
 end

spec/docile_spec.rb

@@ -435,6 +435,36 @@ def dsl_eval_string(string)
         end
       end
     end
+
+    if RUBY_VERSION >= "2.0.0"
+      context "when a DSL method has a keyword argument" do
+        class DSLMethodWithKeywordArgument
+          attr_reader :v0, :v1, :v2
+          class_eval(<<-METHOD)
+            def set(v0, v1:, v2:)
+              @v0 = v0
+              @v1 = v1
+              @v2 = v2
+            end
+          METHOD
+        end
+
+        let(:dsl) do
+          DSLMethodWithKeywordArgument.new
+        end
+
+        it "calls such DSL methods with no stderr output" do
+          # This is to check warnings related to keyword argument is not output.
+          # See: https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/
+          expect { Docile.dsl_eval(dsl) { set(0, v2: 2, v1: 1) } }.
+            not_to output.to_stderr
+
+          expect(dsl.v0).to eq 0
+          expect(dsl.v1).to eq 1
+          expect(dsl.v2).to eq 2
+        end
+      end
+    end
   end
 
   describe ".dsl_eval_with_block_return" do