Ensure that field aliases cannot be used in multi-fields. (#32219)

docs/reference/mapping/types/alias.asciidoc

@@ -74,7 +74,7 @@ field alias to query over multiple target fields in a single clause.
 ==== Unsupported APIs
 
 Writes to field aliases are not supported: attempting to use an alias in an index or update request
-will result in a failure. Likewise, aliases cannot be used as the target of `copy_to`.
+will result in a failure. Likewise, aliases cannot be used as the target of `copy_to` or in multi-fields.
 
 Because alias names are not present in the document source, aliases cannot be used when performing
 source filtering. For example, the following request will return an empty result for `_source`:

server/src/main/java/org/elasticsearch/index/mapper/TypeParsers.java

@@ -230,7 +230,9 @@ public class TypeParsers {
                 } else {
                     throw new MapperParsingException("no type specified for property [" + multiFieldName + "]");
                 }
-                if (type.equals(ObjectMapper.CONTENT_TYPE) || type.equals(ObjectMapper.NESTED_CONTENT_TYPE)) {
+                if (type.equals(ObjectMapper.CONTENT_TYPE)
+                        || type.equals(ObjectMapper.NESTED_CONTENT_TYPE)
+                        || type.equals(FieldAliasMapper.CONTENT_TYPE)) {
                     throw new MapperParsingException("Type [" + type + "] cannot be used in multi field");
                 }
 

server/src/test/java/org/elasticsearch/index/mapper/DocumentMapperParserTests.java

@@ -76,4 +76,28 @@ public class DocumentMapperParserTests extends ESSingleNodeTestCase {
             mapperParser.parse("type", new CompressedXContent(mapping)));
         assertTrue(e.getMessage(), e.getMessage().contains("mapper [foo] of different type"));
     }
+
+    public void testMultiFieldsWithFieldAlias() throws Exception {
+        IndexService indexService = createIndex("test");
+        DocumentMapperParser mapperParser = indexService.mapperService().documentMapperParser();
+        String mapping = Strings.toString(XContentFactory.jsonBuilder().startObject().startObject("type")
+            .startObject("properties")
+                .startObject("field")
+                    .field("type", "text")
+                    .startObject("fields")
+                        .startObject("alias")
+                            .field("type", "alias")
+                            .field("path", "other-field")
+                        .endObject()
+                    .endObject()
+                .endObject()
+                .startObject("other-field")
+                    .field("type", "keyword")
+                .endObject()
+            .endObject()
+        .endObject().endObject());
+        MapperParsingException e = expectThrows(MapperParsingException.class, () ->
+            mapperParser.parse("type", new CompressedXContent(mapping)));
+        assertEquals("Type [alias] cannot be used in multi field", e.getMessage());
+    }
 }