8214113: Switch expressions may have constant type and may be skipped during write

Switch expressions should not be compile-time constants.

Reviewed-by: mcimadamore

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

@@ -1778,9 +1778,6 @@ public class Attr extends JCTree.Visitor {
             if (condTypes.isEmpty()) {
                 return syms.objectType; //TODO: how to handle?
             }
-            if (condTypes.size() == 1) {
-                return condTypes.head;
-            }
             Type first = condTypes.head;
             // If same type, that is the result
             if (condTypes.tail.stream().allMatch(t -> types.isSameType(first, t)))

test/langtools/tools/javac/switchexpr/SwitchExpressionIsNotAConstant.java

@@ -0,0 +1,86 @@
+/*
+ * Copyright (c) 2018, 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 8214113
+ * @summary Verify the switch expression's type does not have a constant attached,
+ *          and so the switch expression is not elided.
+ * @compile --enable-preview --source 12 SwitchExpressionIsNotAConstant.java
+ * @run main/othervm --enable-preview SwitchExpressionIsNotAConstant
+ */
+public class SwitchExpressionIsNotAConstant {
+
+    public static void main(String[] args) {
+        int i = 0;
+        {
+            i = 0;
+            int dummy = 1 + switch (i) {
+                default -> {
+                    i++;
+                    break 1;
+                }
+            };
+            if (i != 1) {
+                throw new IllegalStateException("Side effects missing.");
+            }
+        }
+        {
+            i = 0;
+            int dummy = 1 + switch (i) {
+                case -1 -> 1;
+                default -> {
+                    i++;
+                    break 1;
+                }
+            };
+            if (i != 1) {
+                throw new IllegalStateException("Side effects missing.");
+            }
+        }
+        {
+            i = 0;
+            int dummy = 1 + switch (i) {
+                 default :
+                    i++;
+                    break 1;
+            };
+            if (i != 1) {
+                throw new IllegalStateException("Side effects missing.");
+            }
+        }
+        {
+            i = 0;
+            int dummy = 1 + switch (i) {
+                case -1: break 1;
+                default:
+                    i++;
+                    break 1;
+            };
+            if (i != 1) {
+                throw new IllegalStateException("Side effects missing.");
+            }
+        }
+    }
+
+}