Fix problem with GLB.

framework/src/main/java/org/checkerframework/framework/util/AnnotatedTypes.java

@@ -893,6 +893,17 @@ public static AnnotatedTypeMirror annotatedGLB(
     TypeMirror tm1 = type1.getUnderlyingType();
     TypeMirror tm2 = type2.getUnderlyingType();
     TypeMirror glbJava = TypesUtils.greatestLowerBound(tm1, tm2, atypeFactory.getProcessingEnv());
+    if (glbJava.getKind() == TypeKind.ERROR) {
+      if (type1.getKind() == TypeKind.TYPEVAR) {
+        return type1;
+      }
+      if (type2.getKind() == TypeKind.TYPEVAR) {
+        return type2;
+      }
+      // I think the only way error happens is when one of the types is a typevarible, but just in
+      // case, just return type1.
+      return type1;
+    }
     Types types = atypeFactory.types;
     QualifierHierarchy qualHierarchy = atypeFactory.getQualifierHierarchy();
     if (types.isSubtype(tm1, tm2)) {

framework/tests/all-systems/Issue6810.java

@@ -0,0 +1,11 @@
+import java.util.Set;
+
+public class Issue6810 {
+  static class Box<T extends Box<T>> {}
+
+  abstract static class BoxSet<T extends Box<T>> implements Set<T> {}
+
+  static <E extends Box<E>> void intersect2(BoxSet<? extends E> intersect, Set<E> set2) {
+    intersect.retainAll(set2);
+  }
+}