feat: error on multiple export defaults in module

docs/errors/E0715.md

@@ -0,0 +1,31 @@
+# E0715: cannot use multiple `export default` statements in one module
+
+Modules in JavaScript can use two types of exports: default export and named export. While a module
+can use multiple named exports, it can only use a single default export.
+
+
+```javascript
+export default function foo() {
+  console.log("foo");
+}
+
+export default function bar() {
+  console.log("bar");
+}
+```
+
+
+If you want to export several values from a module, use named exports.
+
+
+```javascript
+function foo(x) {
+  console.log("foo");
+}
+
+function bar(x) {
+  console.log("bar");
+}
+
+export { foo, bar };
+```

po/messages.pot

@@ -2081,6 +2081,14 @@ msgstr ""
 msgid "'async' keyword is not allowed on getters or setters"
 msgstr ""
 
+#: src/quick-lint-js/diag/diagnostic-metadata-generated.cpp
+msgid "cannot use multiple `export default` statements in one module"
+msgstr ""
+
+#: src/quick-lint-js/diag/diagnostic-metadata-generated.cpp
+msgid "export default previously appeared here"
+msgstr ""
+
 #: test/test-diagnostic-formatter.cpp
 #: test/test-vim-qflist-json-diag-reporter.cpp
 msgid "something happened"

src/quick-lint-js/diag/diagnostic-metadata-generated.cpp

@@ -6407,6 +6407,24 @@ const QLJS_CONSTINIT Diagnostic_Info all_diagnostic_infos[] = {
         },
       },
     },
+
+    // Diag_Multiple_Export_Defaults
+    {
+      .code = 715,
+      .severity = Diagnostic_Severity::error,
+      .message_formats = {
+        QLJS_TRANSLATABLE("cannot use multiple `export default` statements in one module"),
+        QLJS_TRANSLATABLE("export default previously appeared here"),
+      },
+      .message_args = {
+        {
+          Diagnostic_Message_Arg_Info(offsetof(Diag_Multiple_Export_Defaults, second_export_default), Diagnostic_Arg_Type::source_code_span),
+        },
+        {
+          Diagnostic_Message_Arg_Info(offsetof(Diag_Multiple_Export_Defaults, first_export_default), Diagnostic_Arg_Type::source_code_span),
+        },
+      },
+    },
 };
 }
 

src/quick-lint-js/diag/diagnostic-metadata-generated.h

@@ -439,10 +439,11 @@ namespace quick_lint_js {
   QLJS_DIAG_TYPE_NAME(Diag_Missing_Comma_Between_Array_Elements) \
   QLJS_DIAG_TYPE_NAME(Diag_Class_Generator_On_Getter_Or_Setter) \
   QLJS_DIAG_TYPE_NAME(Diag_Class_Async_On_Getter_Or_Setter) \
+  QLJS_DIAG_TYPE_NAME(Diag_Multiple_Export_Defaults) \
   /* END */
 // clang-format on
 
-inline constexpr int Diag_Type_Count = 428;
+inline constexpr int Diag_Type_Count = 429;
 
 extern const Diagnostic_Info all_diagnostic_infos[Diag_Type_Count];
 }

src/quick-lint-js/diag/diagnostic-types-2.h

@@ -3307,6 +3307,17 @@ struct Diag_Class_Async_On_Getter_Or_Setter {
   Source_Code_Span async_keyword;
   Source_Code_Span getter_setter_keyword;
 };
+
+struct Diag_Multiple_Export_Defaults {
+  [[qljs::diag("E0715", Diagnostic_Severity::error)]]  //
+  [[qljs::message(
+      "cannot use multiple `export default` statements in one module",
+      ARG(second_export_default))]]  //
+  [[qljs::message("export default previously appeared here",
+                  ARG(first_export_default))]]  //
+  Source_Code_Span second_export_default;
+  Source_Code_Span first_export_default;
+};
 }
 QLJS_WARNING_POP
 

src/quick-lint-js/fe/parse-statement.cpp

@@ -35,6 +35,8 @@ bool Parser::parse_and_visit_module_catching_fatal_parse_errors(
 }
 
 void Parser::parse_and_visit_module(Parse_Visitor_Base &v) {
+  QLJS_ASSERT(
+      !this->first_export_default_statement_default_keyword_.has_value());
   bool done = false;
   Parse_Statement_Options statement_options = {
       .possibly_followed_by_another_statement = true,
@@ -1011,6 +1013,17 @@ void Parser::parse_and_visit_export(Parse_Visitor_Base &v,
   switch (this->peek().type) {
     // export default class C {}
   case Token_Type::kw_default:
+    if (this->first_export_default_statement_default_keyword_.has_value()) {
+      this->diag_reporter_->report(Diag_Multiple_Export_Defaults{
+          .second_export_default = this->peek().span(),
+          .first_export_default =
+              *this->first_export_default_statement_default_keyword_,
+      });
+    } else {
+      this->first_export_default_statement_default_keyword_ =
+          this->peek().span();
+    }
+
     this->is_current_typescript_namespace_non_empty_ = true;
     if (this->in_typescript_namespace_or_module_.has_value() &&
         !this->in_typescript_module_) {

src/quick-lint-js/fe/parse.h

@@ -545,6 +545,11 @@ class Parser {
   void parse_and_visit_named_exports_for_typescript_type_only_import(
       Parse_Visitor_Base &v, Source_Code_Span type_keyword);
 
+  // If set, refers to the first `export default` statement in this module. A
+  // module cannot contain more than one `export default`.
+  std::optional<Source_Code_Span>
+      first_export_default_statement_default_keyword_ = std::nullopt;
+
   struct Parse_Export_Options {
     TypeScript_Declare_Context declare_context;
 

src/quick-lint-js/i18n/translation-table-generated.cpp

@@ -206,6 +206,7 @@ const Translation_Table translation_data = {
         {0, 0, 0, 67, 0, 53},                //
         {0, 0, 0, 0, 0, 37},                 //
         {0, 0, 0, 0, 0, 43},                 //
+        {0, 0, 0, 0, 0, 62},                 //
         {0, 0, 0, 50, 0, 47},                //
         {72, 31, 71, 68, 56, 61},            //
         {34, 30, 0, 46, 0, 40},              //
@@ -259,7 +260,8 @@ const Translation_Table translation_data = {
         {33, 27, 36, 45, 0, 35},             //
         {39, 42, 0, 49, 0, 41},              //
         {24, 24, 0, 24, 0, 24},              //
-        {22, 22, 42, 22, 40, 22},            //
+        {0, 0, 0, 0, 0, 22},                 //
+        {22, 22, 42, 22, 40, 40},            //
         {32, 30, 35, 26, 30, 29},            //
         {0, 0, 0, 27, 0, 32},                //
         {35, 45, 38, 53, 33, 46},            //
@@ -1998,6 +2000,7 @@ const Translation_Table translation_data = {
         u8"cannot update variable with '{0}' while declaring it\0"
         u8"cannot use '...' on 'this' parameter\0"
         u8"cannot use 'declare' keyword with 'import'\0"
+        u8"cannot use multiple `export default` statements in one module\0"
         u8"cannot use type directly in its own definition\0"
         u8"catch variable can only be typed as '*', 'any', or 'unknown'\0"
         u8"character is not allowed in identifiers\0"
@@ -2052,6 +2055,7 @@ const Translation_Table translation_data = {
         u8"expected variable name for 'import'-'as'\0"
         u8"expected {1:headlinese}\0"
         u8"expected {1:singular}\0"
+        u8"export default previously appeared here\0"
         u8"exporting requires 'default'\0"
         u8"exporting requires '{{' and '}'\0"
         u8"extra ',' is not allowed between enum members\0"

src/quick-lint-js/i18n/translation-table-generated.h

@@ -18,8 +18,8 @@ namespace quick_lint_js {
 using namespace std::literals::string_view_literals;
 
 constexpr std::uint32_t translation_table_locale_count = 5;
-constexpr std::uint16_t translation_table_mapping_table_size = 522;
-constexpr std::size_t translation_table_string_table_size = 79941;
+constexpr std::uint16_t translation_table_mapping_table_size = 524;
+constexpr std::size_t translation_table_string_table_size = 80043;
 constexpr std::size_t translation_table_locale_table_size = 35;
 
 QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
@@ -220,6 +220,7 @@ QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
           "cannot update variable with '{0}' while declaring it"sv,
           "cannot use '...' on 'this' parameter"sv,
           "cannot use 'declare' keyword with 'import'"sv,
+          "cannot use multiple `export default` statements in one module"sv,
           "cannot use type directly in its own definition"sv,
           "catch variable can only be typed as '*', 'any', or 'unknown'"sv,
           "character is not allowed in identifiers"sv,
@@ -274,6 +275,7 @@ QLJS_CONSTEVAL std::uint16_t translation_table_const_look_up(
           "expected variable name for 'import'-'as'"sv,
           "expected {1:headlinese}"sv,
           "expected {1:singular}"sv,
+          "export default previously appeared here"sv,
           "exporting requires 'default'"sv,
           "exporting requires '{{' and '}'"sv,
           "extra ',' is not allowed between enum members"sv,

src/quick-lint-js/i18n/translation-table-test-generated.h

@@ -27,7 +27,7 @@ struct Translated_String {
 };
 
 // clang-format off
-inline const Translated_String test_translation_table[521] = {
+inline const Translated_String test_translation_table[523] = {
     {
         "\"global-groups\" entries must be strings"_translatable,
         u8"\"global-groups\" entries must be strings",
@@ -2162,6 +2162,17 @@ inline const Translated_String test_translation_table[521] = {
             u8"cannot use 'declare' keyword with 'import'",
         },
     },
+    {
+        "cannot use multiple `export default` statements in one module"_translatable,
+        u8"cannot use multiple `export default` statements in one module",
+        {
+            u8"cannot use multiple `export default` statements in one module",
+            u8"cannot use multiple `export default` statements in one module",
+            u8"cannot use multiple `export default` statements in one module",
+            u8"cannot use multiple `export default` statements in one module",
+            u8"cannot use multiple `export default` statements in one module",
+        },
+    },
     {
         "cannot use type directly in its own definition"_translatable,
         u8"cannot use type directly in its own definition",
@@ -2756,6 +2767,17 @@ inline const Translated_String test_translation_table[521] = {
             u8"expected {1:singular}",
         },
     },
+    {
+        "export default previously appeared here"_translatable,
+        u8"export default previously appeared here",
+        {
+            u8"export default previously appeared here",
+            u8"export default previously appeared here",
+            u8"export default previously appeared here",
+            u8"export default previously appeared here",
+            u8"export default previously appeared here",
+        },
+    },
     {
         "exporting requires 'default'"_translatable,
         u8"exporting requires 'default'",

test/test-parse-module.cpp

@@ -162,6 +162,24 @@ TEST_F(Test_Parse_Module, export_default) {
                           }));
   }
 
+  {
+    Spy_Visitor p = test_parse_and_visit_module(
+        u8"export default class A {} export default class B {}"_sv,  //
+        u8"       ^^^^^^^ Diag_Multiple_Export_Defaults.first_export_default\n"
+        u8"                                 ^^^^^^^ .second_export_default"_diag);
+    EXPECT_THAT(p.visits, ElementsAreArray({
+                              "visit_enter_class_scope",       // A
+                              "visit_enter_class_scope_body",  //
+                              "visit_exit_class_scope",        //
+                              "visit_variable_declaration",    //
+                              "visit_enter_class_scope",       // B
+                              "visit_enter_class_scope_body",  //
+                              "visit_exit_class_scope",        //
+                              "visit_variable_declaration",    //
+                              "visit_end_of_module",
+                          }));
+  }
+
   {
     Spy_Visitor p = test_parse_and_visit_statement(
         u8"export default async (a) => b;"_sv, no_diags, javascript_options);