Keep window specifications parseable inside lookahead

- The base window name is a choice rather than an optional, so that
  lookahead scans through a window specification keep its check:
  `JSON_VALUE(... DEFAULT sum(a) OVER (PARTITION BY b) ON EMPTY)` parses
  again.
- Only an OVER window may extend a named window, not WITHIN GROUP.
- QUALIFY after WINDOW requires a WINDOW clause.

Author: guizmaii

src/main/jjtree/net/sf/jsqlparser/parser/JSqlParserCC.jjt

@@ -8167,10 +8167,10 @@ PlainSelect PlainSelect() #PlainSelect:
         windowName = RelObjectName() <K_AS> winDef = windowDefinition() { List<WindowDefinition> winDefs = new ArrayList<WindowDefinition>(); winDefs.add(winDef.withWindowName(windowName)); }
         ( LOOKAHEAD(2) "," windowName = RelObjectName() <K_AS> winDef = windowDefinition() { winDefs.add(winDef.withWindowName(windowName)); } )*
         { plainSelect.setWindowDefinitions(winDefs); }
+        // DuckDB: QUALIFY after WINDOW
+        [ LOOKAHEAD(2, { plainSelect.getQualify() == null }) qualify=Qualify()
+            { plainSelect.setQualify(qualify); plainSelect.setQualifyAfterWindow(true); } ]
     ]
-    // DuckDB: QUALIFY after WINDOW
-    [ LOOKAHEAD(2, { plainSelect.getQualify() == null }) qualify=Qualify()
-        { plainSelect.setQualify(qualify); plainSelect.setQualifyAfterWindow(true); } ]
     [ LOOKAHEAD(<K_ORDER> <K_BY>) orderByElements = OrderByElements() { plainSelect.setOrderByElements(orderByElements); } ]
     [ LOOKAHEAD(2) interpolateElements = InterpolateClause() { plainSelect.setInterpolate(interpolateElements); } ]
     [ LOOKAHEAD(2) forClause = ForClause() {plainSelect.setForClause(forClause);} ]
@@ -13120,7 +13120,13 @@ void windowFun(AnalyticExpression retval):{
     (
         windowName = RelObjectName() { retval.setWindowName(windowName); }
         |
-        winDef = windowDefinition() { retval.setWindowDefinition(winDef); }
+        winDef = windowDefinition()
+        {
+            if (winDef.getBaseWindowName() != null && retval.getType() != AnalyticType.OVER) {
+                throw new ParseException("Only an OVER window may extend a named window");
+            }
+            retval.setWindowDefinition(winDef);
+        }
 
         [
             LOOKAHEAD(2) <K_OVER> "("
@@ -13142,9 +13148,14 @@ WindowDefinition windowDefinition() : {
     String baseWindowName;
 } {
     "("
-    // DuckDB: (w ORDER BY b) extends the named window w
-    [ LOOKAHEAD({ isBaseWindowNameAhead() }) baseWindowName=RelObjectName()
-        { winDef.setBaseWindowName(baseWindowName); } ]
+    // DuckDB: (w ORDER BY b) extends the named window w. A choice, not an optional, so that
+    // lookahead scans through a window specification keep the check
+    (
+        LOOKAHEAD({ isBaseWindowNameAhead() }) baseWindowName=RelObjectName()
+        { winDef.setBaseWindowName(baseWindowName); }
+        |
+        {}
+    )
     [<K_PARTITION> <K_BY>
         (LOOKAHEAD(3) expressionList=ComplexExpressionList()
         | "(" {partitionByBrackets = true;} expressionList=ComplexExpressionList() ")" )

src/test/java/net/sf/jsqlparser/statement/select/DuckDBTest.java

@@ -997,7 +997,10 @@ void testWindowAndFilterForms() throws JSQLParserException {
                 "SELECT sum(a) OVER w2 FROM t WINDOW w AS (PARTITION BY c), w2 AS (w ORDER BY b)",
                 "SELECT sum(a) OVER (ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) FROM t",
                 "SELECT sum(a) FILTER (b > 1) FROM t",
-                "SELECT count(*) FILTER (b > 1) OVER () FROM t"}) {
+                "SELECT count(*) FILTER (b > 1) OVER () FROM t",
+                // window specifications inside syntactic lookahead
+                "SELECT JSON_VALUE(j, '$.a' DEFAULT sum(a) OVER (PARTITION BY b) ON EMPTY) FROM t",
+                "SELECT JSON_VALUE(j, '$.a' DEFAULT sum(a) OVER (w ORDER BY b) ON EMPTY) FROM t"}) {
             TestUtils.assertSqlCanBeParsedAndDeparsed(sqlStr, true);
         }
         PlainSelect select = (PlainSelect) CCJSqlParserUtil.parse(
@@ -1006,6 +1009,10 @@ void testWindowAndFilterForms() throws JSQLParserException {
                 (net.sf.jsqlparser.expression.AnalyticExpression) select.getSelectItem(0)
                         .getExpression();
         Assertions.assertFalse(sum.isFilterWhereKeyword());
+        Assertions.assertThrows(JSQLParserException.class, () -> CCJSqlParserUtil.parse(
+                "SELECT percentile_cont(0.5) WITHIN GROUP (w ORDER BY a) FROM t"));
+        Assertions.assertThrows(JSQLParserException.class,
+                () -> CCJSqlParserUtil.parse("SELECT a FROM t ORDER SIBLINGS BY a QUALIFY x = 1"));
         // PostgreSQL requires WHERE
         Assertions.assertThrows(JSQLParserException.class,
                 () -> CCJSqlParserUtil.parse("SELECT sum(a) FILTER (b > 1) FROM t",