8286797: Guards of constant value false are not permitted

Reviewed-by: vromero

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/Attr.java

@@ -1798,6 +1798,10 @@ public class Attr extends JCTree.Visitor {
                                 bodyEnv.info.scope.leave();
                             }
                             matchBindings = matchBindingsComputer.caseGuard(c, afterPattern, matchBindings);
+
+                            if (TreeInfo.isBooleanWithValue(guard, 0)) {
+                                log.error(guard.pos(), Errors.GuardHasConstantExpressionFalse);
+                            }
                         }
                         boolean unguarded = TreeInfo.unguardedCaseLabel(pat);
                         boolean unconditional =

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

@@ -512,6 +512,9 @@ compiler.err.duplicate.unconditional.pattern=\
 compiler.err.unconditional.pattern.and.default=\
     switch has both an unconditional pattern and a default label
 
+compiler.err.guard.has.constant.expression.false=\
+    this case label has a guard that is a constant expression with value ''false''
+
 # 0: type, 1: type
 compiler.err.constant.label.not.compatible=\
     constant label of type {0} is not compatible with switch selector type {1}

src/jdk.compiler/share/classes/com/sun/tools/javac/tree/TreeInfo.java

@@ -1337,9 +1337,13 @@ public class TreeInfo {
         if (guard == null) {
             return true;
         }
+        return isBooleanWithValue(guard, 1);
+    }
+
+    public static boolean isBooleanWithValue(JCExpression guard, int value) {
         var constValue = guard.type.constValue();
         return constValue != null &&
-               guard.type.hasTag(BOOLEAN) &&
-               ((int) constValue) == 1;
+                guard.type.hasTag(BOOLEAN) &&
+                ((int) constValue) == value;
     }
 }

test/langtools/tools/javac/T8286797.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2022, 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.
+ */
+
+/*
+ * @test
+ * @bug 8286797
+ * @summary  Guards of constant value false are not permitted
+ * @compile/fail/ref=T8286797.out --enable-preview -source ${jdk.version} -XDrawDiagnostics -XDshould-stop.at=FLOW T8286797.java
+ */
+
+public class T8286797 {
+
+    public void testWithConstant(Object o) {
+        switch (o) {
+            case String s when false -> {}
+            default -> {}
+        }
+    }
+
+    public void testWithSimpleName(Object o) {
+        final int x = 0;
+        switch (o) {
+            case String s when x == 42 -> {}
+            default -> {}
+        }
+    }
+}

test/langtools/tools/javac/T8286797.out

@@ -0,0 +1,5 @@
+T8286797.java:35:32: compiler.err.guard.has.constant.expression.false
+T8286797.java:43:34: compiler.err.guard.has.constant.expression.false
+- compiler.note.preview.filename: T8286797.java, DEFAULT
+- compiler.note.preview.recompile
+2 errors

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

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2022, 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.guard.has.constant.expression.false
+// key: compiler.misc.feature.pattern.switch
+// key: compiler.warn.preview.feature.use.plural
+// options: --enable-preview -source ${jdk.version} -Xlint:preview
+
+class GuardHasConstantFalse {
+    private void doSwitch(Object o) {
+        switch (o) {
+            case String s when false -> {}
+            default -> {}
+        }
+    }
+}

test/langtools/tools/javac/patterns/EnumTypeChanges.java

@@ -64,7 +64,7 @@ public class EnumTypeChanges {
         switch (e) {
             case A -> { return "A"; }
             case B -> { return "B"; }
-            case EnumTypeChangesEnum e1 when false -> throw new AssertionError();
+            case EnumTypeChangesEnum e1 when e1 == null -> throw new AssertionError();
             default -> { return "D"; }
         }
     }
@@ -73,7 +73,7 @@ public class EnumTypeChanges {
         return switch (e) {
             case A -> "A";
             case B -> "B";
-            case EnumTypeChangesEnum e1 when false -> throw new AssertionError();
+            case EnumTypeChangesEnum e1 when e1 == null -> throw new AssertionError();
             default -> "D";
         };
     }

test/langtools/tools/javac/patterns/Exhaustiveness.java

@@ -276,10 +276,11 @@ public class Exhaustiveness extends TestRunner {
                    }
                }
                """,
+               "Test.java:6:27: compiler.err.guard.has.constant.expression.false",
                "Test.java:5:16: compiler.err.not.exhaustive",
                "- compiler.note.preview.filename: Test.java, DEFAULT",
                "- compiler.note.preview.recompile",
-               "1 error");
+               "2 errors");
     }
 
     @Test

test/langtools/tools/javac/patterns/RawTypeBindingWarning.java

@@ -18,7 +18,7 @@ public class RawTypeBindingWarning<T> {
             default -> {}
         }
         switch (o) {
-            case RawTypeBindingWarning w when false -> {}
+            case RawTypeBindingWarning w when w == null -> {}
             default -> {}
         }
     }