diff --git a/jOOQ/src/main/java/org/jooq/impl/CustomCondition.java b/jOOQ/src/main/java/org/jooq/impl/CustomCondition.java
index d930e4fefb..8b3b314e46 100644
--- a/jOOQ/src/main/java/org/jooq/impl/CustomCondition.java
+++ b/jOOQ/src/main/java/org/jooq/impl/CustomCondition.java
@@ -45,6 +45,7 @@ import org.jooq.Clause;
 import org.jooq.Condition;
 import org.jooq.Context;
 import org.jooq.impl.QOM.UEmptyCondition;
+import org.jooq.impl.QOM.UOpaque;
 
 /**
  * A base class for custom {@link Condition} implementations in client code.
@@ -59,7 +60,13 @@ import org.jooq.impl.QOM.UEmptyCondition;
  *
  * @author Lukas Eder
  */
-public abstract class CustomCondition extends AbstractCondition implements UEmptyCondition {
+public non-sealed abstract class CustomCondition
+extends
+    AbstractCondition
+implements
+    UEmptyCondition,
+    UOpaque
+{
 
     private static final Clause[] CLAUSES = { CUSTOM };
 
diff --git a/jOOQ/src/main/java/org/jooq/impl/CustomField.java b/jOOQ/src/main/java/org/jooq/impl/CustomField.java
index 9f55e09d81..4f83823ab9 100644
--- a/jOOQ/src/main/java/org/jooq/impl/CustomField.java
+++ b/jOOQ/src/main/java/org/jooq/impl/CustomField.java
@@ -48,6 +48,7 @@ import org.jooq.DataType;
 import org.jooq.Field;
 import org.jooq.Name;
 import org.jooq.impl.QOM.UEmptyField;
+import org.jooq.impl.QOM.UOpaque;
 
 /**
  * A base class for custom {@link Field} implementations in client code.
@@ -62,7 +63,13 @@ import org.jooq.impl.QOM.UEmptyField;
  *
  * @author Lukas Eder
  */
-public abstract class CustomField<T> extends AbstractField<T> implements UEmptyField<T> {
+public non-sealed abstract class CustomField<T>
+extends
+    AbstractField<T>
+implements
+    UEmptyField<T>,
+    UOpaque
+{
 
     private static final Clause[] CLAUSES = { CUSTOM };
 
diff --git a/jOOQ/src/main/java/org/jooq/impl/CustomQueryPart.java b/jOOQ/src/main/java/org/jooq/impl/CustomQueryPart.java
index c0a8249001..2796944a90 100644
--- a/jOOQ/src/main/java/org/jooq/impl/CustomQueryPart.java
+++ b/jOOQ/src/main/java/org/jooq/impl/CustomQueryPart.java
@@ -46,6 +46,7 @@ import org.jooq.Condition;
 import org.jooq.Context;
 import org.jooq.QueryPart;
 import org.jooq.impl.QOM.UEmpty;
+import org.jooq.impl.QOM.UOpaque;
 
 /**
  * A base class for custom {@link QueryPart} implementations in client code.
@@ -72,7 +73,7 @@ import org.jooq.impl.QOM.UEmpty;
  *
  * @author Lukas Eder
  */
-public abstract class CustomQueryPart extends AbstractQueryPart implements UEmpty {
+public non-sealed abstract class CustomQueryPart extends AbstractQueryPart implements UOpaque {
 
     private static final Clause[] CLAUSES = { CUSTOM };
 
diff --git a/jOOQ/src/main/java/org/jooq/impl/CustomStatement.java b/jOOQ/src/main/java/org/jooq/impl/CustomStatement.java
index 31ef6d5a9e..d856dc4531 100644
--- a/jOOQ/src/main/java/org/jooq/impl/CustomStatement.java
+++ b/jOOQ/src/main/java/org/jooq/impl/CustomStatement.java
@@ -105,6 +105,13 @@ package org.jooq.impl;
 
 
 
+
+
+
+
+
+
+
 
 
 
diff --git a/jOOQ/src/main/java/org/jooq/impl/CustomTable.java b/jOOQ/src/main/java/org/jooq/impl/CustomTable.java
index 32e48d5942..5aff6e57e9 100644
--- a/jOOQ/src/main/java/org/jooq/impl/CustomTable.java
+++ b/jOOQ/src/main/java/org/jooq/impl/CustomTable.java
@@ -50,6 +50,7 @@ import org.jooq.Schema;
 import org.jooq.Table;
 import org.jooq.TableRecord;
 import org.jooq.UniqueKey;
+import org.jooq.impl.QOM.UOpaque;
 
 /**
  * A base class for custom {@link Table} implementations in client code.
@@ -74,7 +75,12 @@ import org.jooq.UniqueKey;
  *
  * @author Lukas Eder
  */
-public abstract class CustomTable<R extends TableRecord<R>> extends TableImpl<R> {
+public non-sealed abstract class CustomTable<R extends TableRecord<R>>
+extends
+    TableImpl<R>
+implements
+    UOpaque
+{
 
     protected CustomTable(Name name) {
         super(name);
diff --git a/jOOQ/src/main/java/org/jooq/impl/QOM.java b/jOOQ/src/main/java/org/jooq/impl/QOM.java
index 33f739c3c9..8c25942ea6 100644
--- a/jOOQ/src/main/java/org/jooq/impl/QOM.java
+++ b/jOOQ/src/main/java/org/jooq/impl/QOM.java
@@ -6265,6 +6265,24 @@ public final class QOM {
      */
     interface UTransient extends UEmpty {}
 
+    /**
+     * A marker interface for {@link QueryPart} implementations that are mainly
+     * used to render SQL, but unlike {@link UTransient} parts, can also appear
+     * in user expression trees.
+     */
+    /* sealed */ interface UOpaque
+        extends
+            UEmpty
+        /* permits
+
+
+
+            CustomField,
+            CustomTable,
+            CustomCondition,
+            CustomQueryPart */ 
+    {}
+
     /**
      * A marker interface for {@link QueryPart} implementations whose
      * {@link QueryPart} semantics has not yet been implemented.