Casts to or from a domain type are ignored; warn and document.

Prohibiting this outright would break dumps taken from older versions
that contain such casts, which would create far more pain than is
justified here.

Per report by Jaime Casanova and subsequent discussion.

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">

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;

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);