Throw an exception if aliases are specified as the target of copy_to.

Author: jtibshirani

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.
+will result in a failure. This also precludes aliases from being the target of `copy_to`.
 
 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/DocumentParser.java

@@ -832,18 +832,25 @@ private static void parseCopyFields(ParseContext context, List<String> copyToFie
 
     /** Creates an copy of the current field with given field name and boost */
     private static void parseCopy(String field, ParseContext context) throws IOException {
-        FieldMapper fieldMapper = context.docMapper().mappers().getFieldMapper(field);
-        if (fieldMapper != null) {
-            fieldMapper.parse(context);
+        Mapper mapper = context.docMapper().mappers().getMapper(field);
+        if (mapper != null) {
+            if (mapper instanceof FieldMapper) {
+                ((FieldMapper) mapper).parse(context);
+            } else if (mapper instanceof FieldAliasMapper) {
+                throw new IllegalArgumentException("Cannot copy to a field alias [" + mapper.name() + "].");
+            } else {
+                throw new IllegalStateException("The provided mapper [" + mapper.name() +
+                    "] has an unrecognized type [" + mapper.getClass().getSimpleName() + "].");
+            }
         } else {
             // The path of the dest field might be completely different from the current one so we need to reset it
             context = context.overridePath(new ContentPath(0));
 
             final String[] paths = splitAndValidatePath(field);
             final String fieldName = paths[paths.length-1];
             Tuple<Integer, ObjectMapper> parentMapperTuple = getDynamicParentMapper(context, paths, null);
-            ObjectMapper mapper = parentMapperTuple.v2();
-            parseDynamicValue(context, mapper, fieldName, context.parser().currentToken());
+            ObjectMapper objectMapper = parentMapperTuple.v2();
+            parseDynamicValue(context, objectMapper, fieldName, context.parser().currentToken());
             for (int i = 0; i < parentMapperTuple.v1(); i++) {
                 context.path().remove();
             }

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

@@ -1390,7 +1390,7 @@ public void testBlankFieldNames() throws Exception {
         assertThat(ExceptionsHelper.detailedMessage(err), containsString("field name cannot be an empty string"));
     }
 
-    public void testFieldAlias() throws Exception {
+    public void testWriteToFieldAlias() throws Exception {
         String mapping = Strings.toString(XContentFactory.jsonBuilder()
             .startObject()
                 .startObject("type")
@@ -1419,6 +1419,39 @@ public void testFieldAlias() throws Exception {
         assertEquals("Cannot write to a field alias [alias-field].", exception.getCause().getMessage());
     }
 
+     public void testCopyToFieldAlias() throws Exception {
+        String mapping = Strings.toString(XContentFactory.jsonBuilder()
+            .startObject()
+                .startObject("type")
+                    .startObject("properties")
+                        .startObject("alias-field")
+                            .field("type", "alias")
+                            .field("path", "concrete-field")
+                        .endObject()
+                        .startObject("concrete-field")
+                            .field("type", "keyword")
+                        .endObject()
+                        .startObject("text-field")
+                            .field("type", "text")
+                            .field("copy_to", "alias-field")
+                        .endObject()
+                    .endObject()
+                .endObject()
+            .endObject());
+
+        DocumentMapperParser mapperParser = createIndex("test").mapperService().documentMapperParser();
+        DocumentMapper mapper = mapperParser.parse("type", new CompressedXContent(mapping));
+
+        BytesReference bytes = BytesReference.bytes(XContentFactory.jsonBuilder()
+            .startObject()
+                .field("text-field", "value")
+            .endObject());
+        MapperParsingException exception = expectThrows(MapperParsingException.class,
+            () -> mapper.parse(SourceToParse.source("test", "type", "1", bytes, XContentType.JSON)));
+
+        assertEquals("Cannot copy to a field alias [alias-field].", exception.getCause().getMessage());
+    }
+
     public void testDynamicDottedFieldNameWithFieldAlias() throws Exception {
         String mapping = Strings.toString(XContentFactory.jsonBuilder()
             .startObject()