fix(frontend): 规范一下前端实现

src/sem/Sema.cpp

@@ -48,14 +48,27 @@ void RunSema(SysYParser::CompUnitContext& comp_unit) {
   if (!func || !func->block()) {
     throw std::runtime_error("[sema] 缺少 main 函数定义");
   }
+  if (!func->Ident() || func->Ident()->getText() != "main") {
+    throw std::runtime_error("[sema] 入口函数必须命名为 main");
+  }
 
   SymbolTable table;
+  bool seen_return = false;
 
-  for (auto* stmt : func->block()->stmt()) {
-    if (!stmt) {
+  const auto& items = func->block()->blockItem();
+  if (items.empty()) {
+    throw std::runtime_error("[sema] main 函数不能为空，且必须以 return 结束");
+  }
+
+  for (size_t i = 0; i < items.size(); ++i) {
+    auto* item = items[i];
+    if (!item) {
       continue;
     }
-    if (auto* decl = stmt->varDecl()) {
+    if (seen_return) {
+      throw std::runtime_error("[sema] return 必须是 main 函数中的最后一条语句");
+    }
+    if (auto* decl = item->decl() ? item->decl()->varDecl() : nullptr) {
       const std::string name = decl->Ident()->getText();
       if (table.Contains(name)) {
         throw std::runtime_error("[sema] 重复定义变量: " + name);
@@ -66,10 +79,19 @@ void RunSema(SysYParser::CompUnitContext& comp_unit) {
       table.Add(name);
       continue;
     }
-    if (auto* ret = stmt->returnStmt()) {
+    if (auto* stmt = item->stmt(); stmt && stmt->returnStmt()) {
+      auto* ret = stmt->returnStmt();
       CheckExpr(*ret->exp(), table);
-      break;
+      seen_return = true;
+      if (i + 1 != items.size()) {
+        throw std::runtime_error("[sema] return 必须是 main 函数中的最后一条语句");
+      }
+      continue;
     }
-    throw std::runtime_error("[sema] 暂不支持的语句类型");
+    throw std::runtime_error("[sema] 暂不支持的 blockItem 类型");
+  }
+
+  if (!seen_return) {
+    throw std::runtime_error("[sema] main 函数必须包含 return 语句");
   }
 }