LibJS: Multiple 'default' clauses in switch statement are a syntax error

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/2dbea60fe2c
Pull-request: https://github.com/SerenityOS/serenity/pull/3795

Libraries/LibJS/Parser.cpp

@@ -1519,8 +1519,15 @@ NonnullRefPtr<SwitchStatement> Parser::parse_switch_statement()
 
     NonnullRefPtrVector<SwitchCase> cases;
 
-    while (match(TokenType::Case) || match(TokenType::Default))
+    auto has_default = false;
+    while (match(TokenType::Case) || match(TokenType::Default)) {
+        if (match(TokenType::Default)) {
+            if (has_default)
+                syntax_error("Multiple 'default' clauses in switch statement");
+            has_default = true;
+        }
         cases.append(parse_switch_case());
+    }
 
     consume(TokenType::CurlyClose);
 

Libraries/LibJS/Tests/switch-basic.js

@@ -68,3 +68,11 @@ describe("basic switch tests", () => {
         expect(i).toBe(5);
     });
 });
+
+describe("errors", () => {
+    test("syntax errors", () => {
+        expect("switch () {}").not.toEval();
+        expect("switch (foo) { bar }").not.toEval();
+        expect("switch (foo) { default: default: }").not.toEval();
+    });
+});