LibJS: Defer execution of switch default clause until after case clauses

When we encounter a default clause in a switch statement, we should not
execute it immediately, instead we need to wait until all case clauses
have been executed as a matching case clause can break from the
switch/case.

The code is nowhere close to the spec, so instead of fixing it properly
I just made it slightly worse, but correct. Needs a complete refactor at
some point.

Userland/Libraries/LibJS/AST.cpp

@@ -2409,31 +2409,25 @@ Value ThrowStatement::execute(Interpreter& interpreter, GlobalObject& global_obj
 
 Value SwitchStatement::execute(Interpreter& interpreter, GlobalObject& global_object) const
 {
+ // FIXME: This needs a massive refactoring, ideally once we start using continue, break, and return completions.
+ // Instead of having an optional test expression, SwitchCase should be split into CaseClause and DefaultClause.
+ // https://tc39.es/ecma262/#sec-switch-statement
+
  InterpreterNodeScope node_scope { interpreter, *this };
 
  auto discriminant_result = m_discriminant->execute(interpreter, global_object);
  if (interpreter.exception())
  return {};
 
- bool falling_through = false;
  auto last_value = js_undefined();
 
- for (auto& switch_case : m_cases) {
- if (!falling_through && switch_case.test()) {
- auto test_result = switch_case.test()->execute(interpreter, global_object);
- if (interpreter.exception())
- return {};
- if (!is_strictly_equal(discriminant_result, test_result))
- continue;
- }
- falling_through = true;
-
+ auto execute_switch_case = [&](auto const& switch_case) -> Optional<Value> {
  for (auto& statement : switch_case.consequent()) {
  auto value = statement.execute(interpreter, global_object);
  if (!value.is_empty())
  last_value = value;
  if (interpreter.exception())
- return {};
+ return Value {};
  if (interpreter.vm().should_unwind()) {
  if (interpreter.vm().should_unwind_until(ScopeType::Continuable, m_label)) {
  // No stop_unwind(), the outer loop will handle that - we just need to break out of the switch/case.
@@ -2446,7 +2440,33 @@ Value SwitchStatement::execute(Interpreter& interpreter, GlobalObject& global_ob
  }
  }
  }
+ return {};
+ };
+
+ bool falling_through = false;
+ SwitchCase const* default_switch_case = nullptr;
+ for (auto& switch_case : m_cases) {
+ if (!switch_case.test()) {
+ default_switch_case = &switch_case;
+ falling_through = true;
+ continue;
+ }
+ if (!falling_through) {
+ auto test_result = switch_case.test()->execute(interpreter, global_object);
+ if (interpreter.exception())
+ return {};
+ if (!is_strictly_equal(discriminant_result, test_result))
+ continue;
+ }
+ falling_through = true;
+ if (auto result = execute_switch_case(switch_case); result.has_value())
+ return *result;
+ }
+ if (default_switch_case) {
+ if (auto result = execute_switch_case(*default_switch_case); result.has_value())
+ return *result;
  }
+
  return last_value;
 }
 

Userland/Libraries/LibJS/Tests/switch-default-before-case.js

@@ -0,0 +1,9 @@
+test("default clause before matching case clause", () => {
+ switch (1 + 2) {
+ default:
+ expect().fail();
+ break;
+ case 3:
+ return;
+ }
+});