Convert numerically-indexed objects to arrays in concatenations

config/src/main/java/com/typesafe/config/impl/ConfigConcatenation.java

@@ -84,9 +84,18 @@ final class ConfigConcatenation extends AbstractConfigValue implements Unmergeab
     /**
      * Add left and right, or their merger, to builder.
      */
-    private static void join(ArrayList<AbstractConfigValue> builder,
-            AbstractConfigValue right) {
+    private static void join(ArrayList<AbstractConfigValue> builder, AbstractConfigValue origRight) {
         AbstractConfigValue left = builder.get(builder.size() - 1);
+        AbstractConfigValue right = origRight;
+
+        // check for an object which can be converted to a list
+        // (this will be an object with numeric keys, like foo.0, foo.1)
+        if (left instanceof ConfigObject && right instanceof SimpleConfigList) {
+            left = DefaultTransformer.transform(left, ConfigValueType.LIST);
+        } else if (left instanceof SimpleConfigList && right instanceof ConfigObject) {
+            right = DefaultTransformer.transform(right, ConfigValueType.LIST);
+        }
+
         // Since this depends on the type of two instances, I couldn't think
         // of much alternative to an instanceof chain. Visitors are sometimes
         // used for multiple dispatch but seems like overkill.

config/src/test/scala/com/typesafe/config/impl/PropertiesTest.scala

@@ -10,6 +10,7 @@ import com.typesafe.config.Config
 import com.typesafe.config.ConfigParseOptions
 import com.typesafe.config.ConfigFactory
 import com.typesafe.config.ConfigException
+import com.typesafe.config.ConfigResolveOptions
 
 class PropertiesTest extends TestUtils {
     @Test
@@ -152,4 +153,27 @@ class PropertiesTest extends TestUtils {
         }
         assertTrue("expected exception thrown", e.getMessage.contains("LIST"))
     }
+
+    @Test
+    def makeListWithNumericKeysAndMerge() {
+        import scala.collection.JavaConverters._
+
+        val props = new Properties()
+        props.setProperty("a.0", "0")
+        props.setProperty("a.1", "1")
+        props.setProperty("a.2", "2")
+
+        val conf1 = ConfigFactory.parseProperties(props, ConfigParseOptions.defaults())
+        assertEquals(Seq(0, 1, 2), conf1.getIntList("a").asScala.toSeq)
+
+        val conf2 = ConfigFactory.parseString("""
+                a += 3
+                a += 4
+                a = ${a} [ 5, 6 ]
+                a = [-2, -1] ${a}
+                """)
+        val conf = conf2.withFallback(conf1).resolve()
+
+        assertEquals(Seq(-2, -1, 0, 1, 2, 3, 4, 5, 6), conf.getIntList("a").asScala.toSeq)
+    }
 }