Fix instanceof statement reporting

src/jdk.compiler/share/classes/com/sun/tools/javac/parser/JavacParser.java

@@ -3036,8 +3036,19 @@ List<JCStatement> blockStatement() {
             } else {
                 if (t.getTag() == TYPETEST && allowPatternDeclarations) {
                     t = term2Rest(t, TreeInfo.orPrec);
+
                     accept(SEMI);
-                    return List.of(toP(F.at(pos).TypeTestStatement(((JCInstanceOf) t).expr, ((JCInstanceOf)t ).getPattern())));
+
+                    JCExpression expr = ((JCInstanceOf) t).expr;
+                    JCPattern pattern = ((JCInstanceOf) t).getPattern();
+
+                    if (pattern == null) {
+                        log.error(DiagnosticFlag.SYNTAX, expr, Errors.InstanceofStatementPatternOnly);
+                        return List.of(toP(F.at(pos).Exec(checkExprStat(expr))));
+                    }
+
+                    return List.of(toP(F.at(pos).TypeTestStatement(expr, pattern)));
+
                 } else {
                     // This Exec is an "ExpressionStatement"; it subsumes the terminating semicolon
                     t = checkExprStat(t);

src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties

@@ -1548,6 +1548,9 @@ compiler.misc.varargs.trustme.on.reifiable.varargs=\
 compiler.err.instanceof.reifiable.not.safe=\
     {0} cannot be safely cast to {1}
 
+compiler.err.instanceof.statement.pattern.only=\
+    instanceof as a statement should act as a pattern matching operator and not as a type comparison operator
+
 # 0: symbol
 compiler.misc.varargs.trustme.on.non.varargs.meth=\
     Method {0} is not a varargs method.

test/langtools/tools/javac/diags/examples/InstanceofStatementPatternOnly.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2019, 2020, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+// key: compiler.err.instanceof.statement.pattern.only
+// options: --enable-preview -source ${jdk.version} -Xlint:preview
+
+import java.util.List;
+
+class InstanceofStatementPatternOnly {
+    static void patterns_only(Object point) {
+        point instanceof int;
+    }
+}

test/langtools/tools/javac/patterns/declarations/InstanceOfStatementErrors.java

@@ -9,10 +9,15 @@
 
 public class InstanceOfStatementErrors {
     static void exhaustivity_error1(Object point) {
-        point instanceof Point(var x, var y);
+        point instanceof Point(var x, var y); // error
     }
 
     sealed interface IPoint permits Point {}
     record Point(Integer x, Integer y) implements IPoint { }
     record OPoint(Object x, Object y) { }
+
+    static void patterns_only(Object point) {
+        point instanceof int; // error
+    }
+
 }

test/langtools/tools/javac/patterns/declarations/InstanceOfStatementErrors.out

@@ -1,2 +1,3 @@
+InstanceOfStatementErrors.java:20:9: compiler.err.instanceof.statement.pattern.only
 InstanceOfStatementErrors.java:12:9: compiler.err.not.exhaustive.statement
-1 error
+2 errors