Merge pull request #419 from koic/fix_syntax_error_on_ruby_2_7_0

Fix a `SyntaxError` on Ruby 2.7.0 caused by arguments forwarding syntax

Author: koic

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         entry:
-          - { ruby: '2.7', allowed-failure: false }
+          - { ruby: '2.7.0', allowed-failure: false } # Minimum supported Ruby version.
           - { ruby: '3.0', allowed-failure: false }
           - { ruby: '3.1', allowed-failure: false }
           - { ruby: '3.2', allowed-failure: false }

lib/mcp/server_context.rb

@@ -130,9 +130,14 @@ def notify_elicitation_complete(**kwargs)
       end
     end
 
-    def method_missing(name, ...)
+    # Forward arguments explicitly with `*args, **kwargs, &block` rather than the `...` forwarding syntax.
+    # The gem supports Ruby 2.7.0 (see `required_ruby_version`), but RuboCop's Parser backend only runs on Ruby 2.7.8,
+    # so leading-argument forwarding like `def method_missing(name, ...)` is allowed by the linter even though it
+    # raises a `SyntaxError` on Ruby 2.7.0 through 2.7.2 (it was added in Ruby 2.7.3). Explicit forwarding keeps
+    # this method loadable on Ruby 2.7.0.
+    def method_missing(name, *args, **kwargs, &block)
       if @context.respond_to?(name)
-        @context.public_send(name, ...)
+        @context.public_send(name, *args, **kwargs, &block)
       else
         super
       end

test/mcp/server_context_test.rb

@@ -201,6 +201,20 @@ def context.custom_method
       assert_equal "custom_value", server_context.custom_method
     end
 
+    test "ServerContext forwards positional, keyword, and block arguments to the context" do
+      context = Object.new
+      def context.combine(prefix, suffix:, &block)
+        block.call("#{prefix}-#{suffix}")
+      end
+      progress = Progress.new(notification_target: mock, progress_token: nil)
+
+      server_context = ServerContext.new(context, progress: progress, notification_target: mock)
+
+      result = server_context.combine("a", suffix: "b", &:upcase)
+
+      assert_equal "A-B", result
+    end
+
     test "ServerContext#report_progress works with nil context" do
       progress = mock
       progress.expects(:report).with(50, total: 100, message: nil).once

test/test_helper.rb

@@ -6,6 +6,16 @@
 require "minitest/mock"
 require "mocha/minitest"
 
+# mocha relies on `Hash.ruby2_keywords_hash?`, which is absent on Ruby 2.7.0 and the earlier 2.7.x releases
+# before it was backported. Those versions also lack the flag-setting APIs, so no hash is ever flagged as
+# ruby2_keywords and returning `false` is correct. Without this shim, mocha raises `NoMethodError` and
+# the suite cannot run on Ruby 2.7.0.
+unless Hash.respond_to?(:ruby2_keywords_hash?)
+  def Hash.ruby2_keywords_hash?(_hash)
+    false
+  end
+end
+
 require "active_support"
 require "active_support/test_case"