8306578: Report error if no label given in @see and {@link} when no default is available

Reviewed-by: jjg

Author: hns

src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/TagletWriterImpl.java

@@ -494,7 +494,7 @@ private Content linkSeeReferenceOutput(Element holder,
             if (labelContent.isEmpty()) {
                 // A non-empty label is required for fragment links as the
                 // reference target does not provide a useful default label.
-                reportWarning.accept("doclet.link.see.no_label", null);
+                htmlWriter.messages.error(ch.getDocTreePath(refTree), "doclet.link.see.no_label");
                 return invalidTagOutput(resources.getText("doclet.link.see.no_label"),
                         Optional.of(refSignature));
             }

test/langtools/jdk/javadoc/doclet/testSeeLinkAnchor/TestSeeLinkAnchor.java

@@ -23,7 +23,7 @@
 
 /*
  * @test
- * @bug 8200337 8307377
+ * @bug 8200337 8307377 8306578
  * @summary Generalize see and link tags for user-defined anchors
  * @library /tools/lib ../../lib
  * @modules
@@ -140,14 +140,14 @@ public void testMissingLabel(Path base) throws Exception {
                 "--no-platform-links",
                 "nolabel");
 
-        checkExit(Exit.OK);
+        checkExit(Exit.ERROR);
         checkOutput(Output.OUT, true, """
-                    warning: missing reference label
+                    error: missing reference label
                     Link with missing label: {@link ##main}.
                                              ^
                     """,
             """
-                    Class1.java:5: warning: missing reference label
+                    Class1.java:5: error: missing reference label
                     @see ##main
                     ^
                     """);