Correct handling for SwitchStatements without a containing Block (gwtproject#10027)

Also fixed incorrect generics for related code, spotted while auditing
other incomplete typechecks.

Fixes gwtproject#10024

Author: niloc132

dev/core/src/com/google/gwt/dev/jjs/impl/GwtAstBuilder.java

@@ -2812,26 +2812,31 @@ protected JDeclarationStatement pop(LocalDeclaration decl) {
 
     protected JStatement pop(Statement x) {
       JNode pop = (x == null) ? null : pop();
-      if (x instanceof Expression) {
+      if (x instanceof Expression && pop instanceof JExpression) {
         return maybeBoxOrUnbox((JExpression) pop, (Expression) x).makeStatement();
       }
       return (JStatement) pop;
     }
 
-    @SuppressWarnings("unchecked")
     protected <T extends JStatement> List<T> pop(Statement[] statements) {
       if (statements == null) {
         return Collections.emptyList();
       }
-      List<T> result = (List<T>) popList(statements.length);
+      List<T> result = Lists.newArrayList();
+      List<? extends JNode> stack = popList(statements.length);
       int i = 0;
-      for (ListIterator<T> it = result.listIterator(); it.hasNext(); ++i) {
+      for (ListIterator<? extends JNode> it = stack.listIterator(); it.hasNext(); ++i) {
         Object element = it.next();
-        if (element == null) {
-          it.remove();
-        } else if (element instanceof JExpression) {
-          it.set((T)
-              maybeBoxOrUnbox((JExpression) element, (Expression) statements[i]).makeStatement());
+        if (element != null) {
+          if (element instanceof JExpression) {
+            JExpression unboxed = maybeBoxOrUnbox((JExpression) element, (Expression) statements[i]);
+            result.add((T) unboxed.makeStatement());
+          } else if (element instanceof JStatement) {
+            result.add((T) element);
+          } else {
+            throw new IllegalStateException(
+                "Unexpected element type, expected statement or expression: " + element);
+          }
         }
       }
       return result;

dev/core/test/com/google/gwt/dev/jjs/impl/Java17AstTest.java

@@ -397,6 +397,65 @@ public void testInstanceOfPatternMatchingWithConditionalOperator()
           "}");
   }
 
+  // The JDT ast nodes for both switch statements and expressions extend Expression, and specific
+  // ast builder traversals previously. Test switch expressions/statements where statements/blocks
+  // can be encountered.
+  public void testSwitchesWithoutBlocks() throws UnableToCompleteException {
+    compileSnippet("void",
+        "for (int i = 0; i < 10; i++) " +
+            "  switch(i) { " +
+            "    case 1: break;" +
+            "    case 2: " +
+            "    default:" +
+            "  }");
+    compileSnippet("void",
+        "for (int i : new int[] {1, 2, 3, 4}) " +
+            "  switch(i) { " +
+            "    case 1:" +
+            "    case 2:" +
+            "    default:" +
+            "  }");
+    compileSnippet("void",
+        "switch (4) {" +
+            "case 1:" +
+            "  switch (2) {" +
+            "    case 2:" +
+            "    case 4:" +
+            "    default:" +
+            "  }" +
+            "}");
+    compileSnippet("void",
+        "if (true == false) " +
+            "  switch (7) {" +
+            "    case 4: {" +
+            "      break;" +
+            "    }" +
+            "  }" +
+            "else " +
+            "  switch (8) {" +
+            "    case 9:" +
+            "  }");
+
+    compileSnippet("void",
+        "while(true)" +
+            "  switch(99) { " +
+            "    default:" +
+            "  }");
+
+    compileSnippet("void",
+        "do" +
+            "  switch(0) { " +
+            "    default:" +
+            "  }" +
+            "while (false);");
+
+    compileSnippet("void",
+        "foo:" +
+            "  switch(123) { " +
+            "    default:" +
+            "  }");
+  }
+
   @Override
   protected void optimizeJava() {
   }