--- a/test/langtools/tools/javac/tree/JavacTreeScannerTest.java	Wed Jul 31 13:35:42 2019 -0700
+++ b/test/langtools/tools/javac/tree/JavacTreeScannerTest.java	Wed Jul 31 13:39:22 2019 -0700
@@ -102,8 +102,21 @@
 
             error(sourcefile, "differences found");
 
-            if (found.size() != expect.size())
+            if (found.size() != expect.size()) {
                 error("Size mismatch; found: " + found.size() + ", expected: " + expect.size());
+                Set<JCTree> notFound = new HashSet<>(expect);
+                notFound.removeAll(found);
+                if (!notFound.isEmpty()) {
+                    System.err.println("found by reflective access to the AST, but not found in the scanner API:");
+                    notFound.forEach(t -> System.err.println(trim(t, 64)));
+                }
+                Set<JCTree> notExpected = new HashSet<>(found);
+                notExpected.removeAll(expect);
+                if (!notExpected.isEmpty()) {
+                    System.err.println("found in the scanner API, but not found by reflective access to the AST:");
+                    notExpected.forEach(t -> System.err.println(trim(t, 64)));
+                }
+            }
 
             Set<JCTree> missing = new HashSet<JCTree>();
             missing.addAll(expect);