Fix MERGE's test for unreachable WHEN clauses.

The former code would only detect an unreachable WHEN clause if it had
an AND condition. Fix, so that unreachable unconditional WHEN clauses
are also detected.

Back-patch to v15, where MERGE was added.

Discussion: https://postgr.es/m/CAEZATCVQ=7E2z4cSBB49jjeGGsB6WeoYQY32NDeSvcHiLUZ=ow@mail.gmail.com

src/backend/parser/parse_merge.c

@@ -155,12 +155,12 @@ transformMergeStmt(ParseState *pstate, MergeStmt *stmt)
 		/*
 		 * Check for unreachable WHEN clauses
 		 */
-		if (mergeWhenClause->condition == NULL)
-			is_terminal[when_type] = true;
-		else if (is_terminal[when_type])
+		if (is_terminal[when_type])
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
 					 errmsg("unreachable WHEN clause specified after unconditional WHEN clause")));
+		if (mergeWhenClause->condition == NULL)
+			is_terminal[when_type] = true;
 	}
 
 	/*

src/test/regress/expected/merge.out

@@ -659,7 +659,7 @@ USING source AS s
 ON t.tid = s.sid
 WHEN MATCHED THEN /* Terminal WHEN clause for MATCHED */
 	DELETE
-WHEN MATCHED AND s.delta > 0 THEN
+WHEN MATCHED THEN
 	UPDATE SET balance = t.balance - s.delta;
 ERROR:  unreachable WHEN clause specified after unconditional WHEN clause
 ROLLBACK;

src/test/regress/sql/merge.sql

@@ -438,7 +438,7 @@ USING source AS s
 ON t.tid = s.sid
 WHEN MATCHED THEN /* Terminal WHEN clause for MATCHED */
 	DELETE
-WHEN MATCHED AND s.delta > 0 THEN
+WHEN MATCHED THEN
 	UPDATE SET balance = t.balance - s.delta;
 ROLLBACK;