Improve error for 'class C {'

If a class body is incomplete, quick-lint-js fails with an unexpected
token error. Improve the error by special-casing end-of-file when
parsing class members.

Author: strager

docs/CHANGELOG.md

@@ -6,6 +6,13 @@ based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 quick-lint-js' version numbers are arbitrary. quick-lint-js does *not* adhere to
 Semantic Versioning.
 
+## Unreleased
+
+### Added
+
+* An incomplete `class` now reports [E0199][] instead of failing with the
+  catch-all [E0054][]. (Thanks to [Dave Churchill][] for reporting.)
+
 ## 2.2.0 (2022-02-17)
 
 [Downloads](https://c.quick-lint-js.com/releases/2.2.0/)
@@ -401,6 +408,7 @@ Beta release.
 [Amir]: https://github.com/ahmafi
 [Christian Mund]: https://github.com/kkkrist
 [Daniel La Rocque]: https://github.com/dlarocque
+[Dave Churchill]: https://www.cs.mun.ca/~dchurchill/
 [David Vasileff]: https://github.com/dav000
 [Erlliam Mejia]: https://github.com/erlliam
 [Himanshu]: https://github.com/singalhimanshu
@@ -463,6 +471,7 @@ Beta release.
 [E0195]: https://quick-lint-js.com/errors/E0195/
 [E0196]: https://quick-lint-js.com/errors/E0196/
 [E0197]: https://quick-lint-js.com/errors/E0197/
+[E0199]: https://quick-lint-js.com/errors/E0199/
 [E0203]: https://quick-lint-js.com/errors/E0203/
 [E0205]: https://quick-lint-js.com/errors/E0205/
 [E0207]: https://quick-lint-js.com/errors/E0207/

docs/errors/E0199.md

@@ -0,0 +1,26 @@
+# E0199: unclosed class; expected '}' by end of file
+
+```config-for-examples
+{
+  "globals": {
+    "doWork": true,
+    "work": true
+  }
+}
+```
+
+Every `{` introducing a class block must have a matching `}` ending a class
+block. It is a syntax error to omit the `}`:
+
+    class Banana {
+      peel() {
+        throw new Error("Bananas can't peel themselves!");
+      }
+
+To fix this error, write a matching `}`:
+
+    class Banana {
+      peel() {
+        throw new Error("Bananas can't peel themselves!");
+      }
+    }

src/quick-lint-js/error.h

@@ -1223,6 +1223,11 @@
       { source_code_span comment_open; },                                      \
       ERROR(QLJS_TRANSLATABLE("unclosed block comment"), comment_open))        \
                                                                                \
+  QLJS_ERROR_TYPE(                                                             \
+      error_unclosed_class_block, "E0199", { source_code_span block_open; },   \
+      ERROR(QLJS_TRANSLATABLE("unclosed class; expected '}' by end of file"),  \
+            block_open))                                                       \
+                                                                               \
   QLJS_ERROR_TYPE(                                                             \
       error_unclosed_code_block, "E0134", { source_code_span block_open; },    \
       ERROR(QLJS_TRANSLATABLE(                                                 \

src/quick-lint-js/parse-statement-inl.h

@@ -1198,10 +1198,17 @@ template <QLJS_PARSE_VISITOR Visitor>
 void parser::parse_and_visit_class_body(Visitor &v) {
   class_guard g(this, std::exchange(this->in_class_, true));
 
+  source_code_span left_curly_span = this->peek().span();
   this->skip();
 
   while (this->peek().type != token_type::right_curly) {
     this->parse_and_visit_class_member(v);
+    if (this->peek().type == token_type::end_of_file) {
+      this->error_reporter_->report(error_unclosed_class_block{
+          .block_open = left_curly_span,
+      });
+      return;
+    }
   }
 
   QLJS_PARSER_UNIMPLEMENTED_IF_NOT_TOKEN(token_type::right_curly);
@@ -1521,6 +1528,11 @@ void parser::parse_and_visit_class_member(Visitor &v) {
     this->skip();
     break;
 
+  // class C {  // Invalid.
+  case token_type::end_of_file:
+    // An error is reported by parse_and_visit_class_body.
+    break;
+
   default:
     QLJS_PARSER_UNIMPLEMENTED();
     break;

test/test-parse-class.cpp

@@ -129,6 +129,39 @@ TEST(test_parse, class_statement_requires_a_body) {
   }
 }
 
+TEST(test_parse, unclosed_class_statement) {
+  {
+    spy_visitor v;
+    padded_string code(u8"class C { "_sv);
+    parser p(&code, &v);
+    EXPECT_TRUE(p.parse_and_visit_statement(v));
+    EXPECT_THAT(v.visits, ElementsAre("visit_variable_declaration",  // C
+                                      "visit_enter_class_scope",     //
+                                      "visit_exit_class_scope"));
+    EXPECT_THAT(v.errors, ElementsAre(ERROR_TYPE_OFFSETS(
+                              &code, error_unclosed_class_block,  //
+                              block_open, strlen(u8"class C "), u8"{")));
+  }
+
+  {
+    spy_visitor v;
+    padded_string code(u8"class C { method() {} "_sv);
+    parser p(&code, &v);
+    EXPECT_TRUE(p.parse_and_visit_statement(v));
+    EXPECT_THAT(v.visits,
+                ElementsAre("visit_variable_declaration",       // C
+                            "visit_enter_class_scope",          // C
+                            "visit_property_declaration",       // method
+                            "visit_enter_function_scope",       // method
+                            "visit_enter_function_scope_body",  // method
+                            "visit_exit_function_scope",        // method
+                            "visit_exit_class_scope"));         // C
+    EXPECT_THAT(v.errors, ElementsAre(ERROR_TYPE_OFFSETS(
+                              &code, error_unclosed_class_block,  //
+                              block_open, strlen(u8"class C "), u8"{")));
+  }
+}
+
 TEST(test_parse, class_statement_with_odd_heritage) {
   {
     // TODO(strager): Should this report errors?