Fix coerce_to_target_type for coerce_type's klugy handling of COLLATE.

Because coerce_type recurses into the argument of a CollateExpr,
coerce_to_target_type's longstanding code for detecting whether coerce_type
had actually done anything (to wit, returned a different node than it
passed in) was broken in 9.1.  This resulted in unexpected failures in
hide_coercion_node; which was not the latter's fault, since it's critical
that we never call it on anything that wasn't inserted by coerce_type.
(Else we might decide to "hide" a user-written function call.)

Fix by removing and replacing the CollateExpr in coerce_to_target_type
itself.  This is all pretty ugly but I don't immediately see a way to make
it nicer.

Per report from Jean-Yves F. Barbier.

src/backend/parser/parse_coerce.c

@@ -79,10 +79,24 @@ coerce_to_target_type(ParseState *pstate, Node *expr, Oid exprtype,
 					  int location)
 {
 	Node	   *result;
+	Node	   *origexpr;
 
 	if (!can_coerce_type(1, &exprtype, &targettype, ccontext))
 		return NULL;
 
+	/*
+	 * If the input has a CollateExpr at the top, strip it off, perform the
+	 * coercion, and put a new one back on.  This is annoying since it
+	 * duplicates logic in coerce_type, but if we don't do this then it's too
+	 * hard to tell whether coerce_type actually changed anything, and we
+	 * *must* know that to avoid possibly calling hide_coercion_node on
+	 * something that wasn't generated by coerce_type.  Note that if there are
+	 * multiple stacked CollateExprs, we just discard all but the topmost.
+	 */
+	origexpr = expr;
+	while (expr && IsA(expr, CollateExpr))
+		expr = (Node *) ((CollateExpr *) expr)->arg;
+
 	result = coerce_type(pstate, expr, exprtype,
 						 targettype, targettypmod,
 						 ccontext, cformat, location);
@@ -98,6 +112,18 @@ coerce_to_target_type(ParseState *pstate, Node *expr, Oid exprtype,
 								(cformat != COERCE_IMPLICIT_CAST),
 								(result != expr && !IsA(result, Const)));
 
+	if (expr != origexpr)
+	{
+		/* Reinstall top CollateExpr */
+		CollateExpr *coll = (CollateExpr *) origexpr;
+		CollateExpr *newcoll = makeNode(CollateExpr);
+
+		newcoll->arg = (Expr *) result;
+		newcoll->collOid = coll->collOid;
+		newcoll->location = coll->location;
+		result = (Node *) newcoll;
+	}
+
 	return result;
 }
 
@@ -318,7 +344,7 @@ coerce_type(ParseState *pstate, Node *node,
 		 * If we have a COLLATE clause, we have to push the coercion
 		 * underneath the COLLATE.	This is really ugly, but there is little
 		 * choice because the above hacks on Consts and Params wouldn't happen
-		 * otherwise.
+		 * otherwise.  This kluge has consequences in coerce_to_target_type.
 		 */
 		CollateExpr *coll = (CollateExpr *) node;
 		CollateExpr *newcoll = makeNode(CollateExpr);

src/test/regress/expected/collate.out

@@ -574,6 +574,9 @@ ALTER TABLE collate_test22 ADD FOREIGN KEY (f2) REFERENCES collate_test20;
 RESET enable_seqscan;
 RESET enable_hashjoin;
 RESET enable_nestloop;
+-- 9.1 bug with useless COLLATE in an expression subject to length coercion
+CREATE TEMP TABLE vctable (f1 varchar(25));
+INSERT INTO vctable VALUES ('foo' COLLATE "C");
 --
 -- Clean up.  Many of these table names will be re-used if the user is
 -- trying to run any platform-specific collation tests later, so we

src/test/regress/sql/collate.sql

@@ -214,6 +214,11 @@ RESET enable_seqscan;
 RESET enable_hashjoin;
 RESET enable_nestloop;
 
+-- 9.1 bug with useless COLLATE in an expression subject to length coercion
+
+CREATE TEMP TABLE vctable (f1 varchar(25));
+INSERT INTO vctable VALUES ('foo' COLLATE "C");
+
 --
 -- Clean up.  Many of these table names will be re-used if the user is
 -- trying to run any platform-specific collation tests later, so we