Fix Promise#then.

lib/concurrent/promise.rb

@@ -307,7 +307,16 @@ def self.execute(opts = {}, &block)
     # @yield The block operation to be performed asynchronously.
     #
     # @return [Promise] the new promise
-    def then(rescuer = nil, executor = @executor, &block)
+    def then(*args, &block)
+      if args.last.is_a?(::Hash)
+        executor = args.pop[:executor]
+        rescuer = args.first
+      else
+        rescuer, executor = args
+      end
+
+      executor ||= @executor
+
       raise ArgumentError.new('rescuers and block are both missing') if rescuer.nil? && !block_given?
       block = Proc.new { |result| result } unless block_given?
       child = Promise.new(

spec/concurrent/promise_spec.rb

@@ -225,6 +225,13 @@ def get_ivar_from_args(opts)
         expect(child).not_to be empty_root
         expect(child.instance_variable_get(:@executor)).to be(new_executor)
       end
+
+      it 'supports setting the executor using a named parameter' do
+        new_executor = Concurrent::SingleThreadExecutor.new
+        child = empty_root.then(executor: new_executor) { nil }
+        expect(child.instance_variable_get(:@executor)).to be(new_executor)
+      end
+
       it 'should have block or rescuers' do
         expect { empty_root.then }.to raise_error(ArgumentError)
       end