Switch exprs must be transformed rather than wrapped in statements (#10122)

Fixes #10121

Author: niloc132

dev/core/src/com/google/gwt/dev/jjs/ast/JExpression.java

@@ -31,7 +31,7 @@ public JExpression(SourceInfo info) {
   /**
    * Returns a statement that executes this expression.
    */
-  public final JExpressionStatement makeStatement() {
+  public JStatement makeStatement() {
     return new JExpressionStatement(getSourceInfo(), this);
   }
 

dev/core/src/com/google/gwt/dev/jjs/ast/JProgram.java

@@ -209,7 +209,7 @@ public int compare(JArrayType o1, JArrayType o2) {
   /**
    * Helper to create an assignment, used to initialize fields, etc.
    */
-  public static JExpressionStatement createAssignmentStmt(SourceInfo info, JExpression lhs,
+  public static JStatement createAssignmentStmt(SourceInfo info, JExpression lhs,
       JExpression rhs) {
     return createAssignment(info, lhs, rhs).makeStatement();
   }

dev/core/src/com/google/gwt/dev/jjs/ast/JSwitchExpression.java

@@ -62,4 +62,9 @@ public void traverse(JVisitor visitor, Context ctx) {
   public JType getType() {
     return type;
   }
+
+  @Override
+  public JStatement makeStatement() {
+    return new JSwitchStatement(this);
+  }
 }

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

@@ -700,16 +700,7 @@ public JsNode transformDoStatement(JDoStatement doStatement) {
 
     @Override
     public JsNode transformExpressionStatement(JExpressionStatement statement) {
-      if (statement.getExpr() instanceof JSwitchExpression) {
-        if (statement.getExpr().getType() == JPrimitiveType.VOID) {
-          return transformSwitchStatement(new JSwitchStatement((JSwitchExpression)
-                  statement.getExpr()));
-        } else {
-          throw new IllegalStateException("top-level switch expr");
-        }
-      } else {
-        return transform(statement.getExpr()).makeStmt();
-      }
+      return transform(statement.getExpr()).makeStmt();
     }
 
     @Override

user/test-super/com/google/gwt/dev/jjs/super/com/google/gwt/dev/jjs/test/Java17Test.java

@@ -548,4 +548,46 @@ private static String arrowWithStatement() {
     };
     return "success";
   }
+
+
+  // https://github.com/gwtproject/gwt/issues/10121
+  public void testUnusedSwitchExprResult() {
+    // When the result of a switch expression is unused, the switch expression is made into
+    // a statement. When this is done by wrapping the SwitchExpression in an ExpressionStatement,
+    // it causes an error when generating JavaScript.
+    int unused1 = switch(0) {
+      case 0 -> 1;
+      case 1 -> 2;
+      default -> 3;
+    };
+    boolean called = false;
+    int unused2 = switch(0) {
+      case 0 -> {
+        called = true;
+        assertTrue(true);
+        yield 1;
+      }
+      case 1 -> {
+        fail();
+        yield 2;
+      }
+      default -> {
+        fail();
+        yield 3;
+      }
+    };
+    assertTrue(called);
+
+    Void unused3 = switch(0) {
+      case 0 -> throwIfFalse(true);
+      case 1 -> throwIfFalse(false);
+      default -> throwIfFalse(false);
+    };
+  }
+  private static Void throwIfFalse(boolean condition) {
+    if (!condition) {
+      fail();
+    }
+    return null;
+  }
 }

user/test/com/google/gwt/dev/jjs/test/Java17Test.java

@@ -121,10 +121,15 @@ public void testSwitchExprInlining() {
   public void testInlinedStringConstantsInCase() {
     assertFalse(isGwtSourceLevel17());
   }
+
   public void testCaseArrowLabelsVoidExpression() {
     assertFalse(isGwtSourceLevel17());
   }
 
+  public void testUnusedSwitchExprResult() {
+    assertFalse(isGwtSourceLevel17());
+  }
+
   private boolean isGwtSourceLevel17() {
     return JUnitShell.getCompilerOptions().getSourceLevel().compareTo(SourceLevel.JAVA17) >= 0;
   }