diff --git a/doc/src/sgml/ref/create_cast.sgml b/doc/src/sgml/ref/create_cast.sgml
index 964cbf4d41..29ea298a46 100644
--- a/doc/src/sgml/ref/create_cast.sgml
+++ b/doc/src/sgml/ref/create_cast.sgml
@@ -288,6 +288,11 @@ SELECT CAST ( 2 AS numeric ) + 4.0;
    convert between data types and a second to apply the modifier.
   </para>
 
+  <para>
+   A cast to or from a domain type currently has no effect.  Casting
+   to or from a domain uses the casts associated with its underlying type.
+  </para>
+
  </refsect1>
 
  <refsect1 id="sql-createcast-notes">
diff --git a/src/backend/commands/functioncmds.c b/src/backend/commands/functioncmds.c
index 4125b97e89..5f1c19eb37 100644
--- a/src/backend/commands/functioncmds.c
+++ b/src/backend/commands/functioncmds.c
@@ -1517,6 +1517,17 @@ CreateCast(CreateCastStmt *stmt)
 		aclcheck_error(aclresult, ACL_KIND_TYPE,
 					   format_type_be(targettypeid));
 
+	/* Domains are allowed for historical reasons, but we warn */
+	if (sourcetyptype == TYPTYPE_DOMAIN)
+		ereport(WARNING,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("cast will be ignored because the source data type is a domain")));
+
+	else if (targettyptype == TYPTYPE_DOMAIN)
+		ereport(WARNING,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("cast will be ignored because the target data type is a domain")));
+
 	/* Detemine the cast method */
 	if (stmt->func != NULL)
 		castmethod = COERCION_METHOD_FUNCTION;
diff --git a/src/test/regress/expected/privileges.out b/src/test/regress/expected/privileges.out
index a816cce1b1..dfaf3a7a7e 100644
--- a/src/test/regress/expected/privileges.out
+++ b/src/test/regress/expected/privileges.out
@@ -571,6 +571,7 @@ CREATE DOMAIN testdomain2b AS testdomain1;
 CREATE DOMAIN testdomain3b AS int;
 CREATE FUNCTION castfunc(int) RETURNS testdomain3b AS $$ SELECT $1::testdomain3b $$ LANGUAGE SQL;
 CREATE CAST (testdomain1 AS testdomain3b) WITH FUNCTION castfunc(int);
+WARNING:  cast will be ignored because the source data type is a domain
 CREATE FUNCTION testfunc5b(a testdomain1) RETURNS int LANGUAGE SQL AS $$ SELECT $1 $$;
 CREATE FUNCTION testfunc6b(b int) RETURNS testdomain1 LANGUAGE SQL AS $$ SELECT $1::testdomain1 $$;
 CREATE OPERATOR !! (PROCEDURE = testfunc5b, RIGHTARG = testdomain1);