Fix array coercion expressions to ensure that the correct volatility is

seen by code inspecting the expression.  The best way to do this seems
to be to drop the original representation as a function invocation, and
instead make a special expression node type that represents applying
the element-type coercion function to each array element.  In this way
the element function is exposed and will be checked for volatility.
Per report from Guillaume Smet.

src/include/nodes/primnodes.h

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/primnodes.h,v 1.128 2007/03/17 00:11:05 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/primnodes.h,v 1.129 2007/03/27 23:21:12 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -550,6 +550,29 @@ typedef struct RelabelType
 	CoercionForm relabelformat; /* how to display this node */
 } RelabelType;
 
+/* ----------------
+ * ArrayCoerceExpr
+ *
+ * ArrayCoerceExpr represents a type coercion from one array type to another,
+ * which is implemented by applying the indicated element-type coercion
+ * function to each element of the source array.  If elemfuncid is InvalidOid
+ * then the element types are binary-compatible, but the coercion still
+ * requires some effort (we have to fix the element type ID stored in the
+ * array header).
+ * ----------------
+ */
+
+typedef struct ArrayCoerceExpr
+{
+	Expr		xpr;
+	Expr	   *arg;			/* input expression (yields an array) */
+	Oid			elemfuncid;		/* OID of element coercion function, or 0 */
+	Oid			resulttype;		/* output type of coercion (an array type) */
+	int32		resulttypmod;	/* output typmod (also element typmod) */
+	bool		isExplicit;		/* conversion semantics flag to pass to func */
+	CoercionForm coerceformat;	/* how to display this node */
+} ArrayCoerceExpr;
+
 /* ----------------
  * ConvertRowtypeExpr
  *