fix(typescript): merge 'export default interface' with 'export defaul…

…t class'

TypeScript performs declaration merging on default exports. Have us do
the same to avoid falsely reporting duplicate export errors.

docs/CHANGELOG.md

@@ -17,6 +17,9 @@ Semantic Versioning.
 * Detection of multiple `export default` statements ([E0715][]) now also applies
   to `export {... as default};` statements.
 * TypeScript support (still experimental):
+  * `export default` with a class and an interface (triggering declaration
+    merging) no longer fasely reports [E0715][] ("cannot use multiple `export
+    default` statements in one module").
   * `export as namespace` statements are now parsed.
   * Assertion signatures (`function f(param): asserts param`) are now parsed.
   * `case await x:` no longer treats `:` as if it was a type annotation colon in

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

@@ -1015,7 +1015,6 @@ void Parser::parse_and_visit_export(Parse_Visitor_Base &v,
     // export default class C {}
   case Token_Type::kw_default: {
     Source_Code_Span default_keyword = this->peek().span();
-    this->found_default_export(default_keyword);
 
     this->is_current_typescript_namespace_non_empty_ = true;
     if (this->in_typescript_namespace_or_module_.has_value() &&
@@ -1027,6 +1026,11 @@ void Parser::parse_and_visit_export(Parse_Visitor_Base &v,
           });
     }
     this->skip();
+
+    this->found_default_export(default_keyword,
+                               /*is_mergeable_interface=*/this->peek().type ==
+                                   Token_Type::kw_interface);
+
     switch (this->peek().type) {
       // export default async function f() {}
       // export default async () => {}
@@ -1574,15 +1578,29 @@ void Parser::parse_and_visit_export(Parse_Visitor_Base &v,
   }
 }
 
-void Parser::found_default_export(Source_Code_Span default_keyword) {
+void Parser::found_default_export(Source_Code_Span default_keyword,
+                                  bool is_mergeable_interface) {
   if (this->first_export_default_statement_default_keyword_.has_value()) {
-    this->diag_reporter_->report(Diag_Multiple_Export_Defaults{
-        .second_export_default = default_keyword,
-        .first_export_default =
-            *this->first_export_default_statement_default_keyword_,
-    });
+    if (is_mergeable_interface) {
+      // export default class {}  export default interface I {}
+    } else {
+      // export default class {}  export default class {}  // Invalid.
+      this->diag_reporter_->report(Diag_Multiple_Export_Defaults{
+          .second_export_default = default_keyword,
+          .first_export_default =
+              *this->first_export_default_statement_default_keyword_,
+      });
+    }
   } else {
-    this->first_export_default_statement_default_keyword_ = default_keyword;
+    if (is_mergeable_interface) {
+      // export default interface I {}  ...
+      // export default interface I {}  export default class {}  // Merged.
+      // Allow an interface to merge with any other default export.
+      // TODO(#1107): An interface is not always mergeable with anything.
+    } else {
+      // export default class {}  ...
+      this->first_export_default_statement_default_keyword_ = default_keyword;
+    }
   }
 }
 
@@ -4726,7 +4744,8 @@ void Parser::parse_and_visit_named_exports(
             v.visit_variable_type_use(left_name);
           } else if (right_token.type == Token_Type::kw_default) {
             // export {C as default};
-            this->found_default_export(/*default_keyword=*/right_token.span());
+            this->found_default_export(/*default_keyword=*/right_token.span(),
+                                       /*is_mergeable_interface=*/false);
             v.visit_variable_export_default_use(left_name);
           } else {
             // export {C};

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

@@ -568,7 +568,8 @@ class Parser {
       std::optional<Source_Code_Span> typescript_type_only_keyword,
       Vector<Token> *out_exported_bad_tokens);
 
-  void found_default_export(Source_Code_Span default_keyword);
+  void found_default_export(Source_Code_Span default_keyword,
+                            bool is_mergeable_interface);
 
   void parse_and_visit_variable_declaration_statement(
       Parse_Visitor_Base &v,

test/test-parse-module.cpp

@@ -215,6 +215,24 @@ TEST_F(Test_Parse_Module, multiple_default_exports) {
   }
 }
 
+TEST_F(Test_Parse_Module,
+       default_exports_can_be_typescript_declaration_merged) {
+  {
+    Spy_Visitor p = test_parse_and_visit_module(
+        u8"export default class C {}  export default interface I {}"_sv,
+        no_diags, typescript_options);
+  }
+
+  {
+    Spy_Visitor p = test_parse_and_visit_module(
+        u8"export default interface I {}  export default class C {}"_sv,
+        no_diags, typescript_options);
+  }
+
+  // TODO(#1107): There are some cases where declaration merging does not occur
+  // between classes and interfaces.
+}
+
 TEST_F(Test_Parse_Module,
        export_default_with_contextual_keyword_variable_expression) {
   Dirty_Set<String8> variable_names =