Improve error for missing arrow operator

The following code:

    let f = () {}

Should report error_missing_arrow_operator_in_arrow_function
Fix: #461

Author: singalhimanshu

src/parse.cpp

@@ -280,9 +280,19 @@ expression* parser::parse_primary_expression(precedence prec) {
     if (this->peek().type == token_type::right_paren) {
       source_code_span right_paren_span = this->peek().span();
       this->skip();
-      if (this->peek().type == token_type::equal_greater) {
-        this->skip();
+      bool is_arrow_function = this->peek().type == token_type::equal_greater;
+      bool is_arrow_function_without_arrow =
+          this->peek().type == token_type::left_curly;
+      if (is_arrow_function || is_arrow_function_without_arrow) {
         // Arrow function: () => expression-or-block
+        // Arrow function: () { }  // Invalid.
+        if (is_arrow_function) {
+          this->skip();
+        } else {
+          this->error_reporter_->report(
+              error_missing_arrow_operator_in_arrow_function{
+                  .where = this->peek().span()});
+        }
         expression* ast = this->parse_arrow_function_body(
             function_attributes::normal, left_paren_span.begin(),
             /*allow_in_operator=*/prec.in_operator);

src/quick-lint-js/error.h

@@ -666,6 +666,12 @@
             expected_right_square)                                             \
           NOTE(QLJS_TRANSLATABLE("array started here"), left_square))          \
                                                                                \
+  QLJS_ERROR_TYPE(                                                             \
+      error_missing_arrow_operator_in_arrow_function, "E176",                  \
+      { source_code_span where; },                                             \
+      ERROR(QLJS_TRANSLATABLE("missing arrow operator for arrow function"),    \
+            where))                                                            \
+                                                                               \
   QLJS_ERROR_TYPE(                                                             \
       error_missing_arrow_function_parameter_list, "E105",                     \
       { source_code_span arrow; },                                             \

test/test-parse-function.cpp

@@ -975,6 +975,38 @@ TEST(test_parse, arrow_function_with_invalid_parameters) {
   }
 }
 
+TEST(test_parse, arrow_function_expression_without_arrow_operator) {
+  {
+    padded_string code(u8"(() {});"_sv);
+    spy_visitor v;
+    parser p(&code, &v);
+    p.parse_and_visit_module(v);
+    EXPECT_THAT(v.visits, ElementsAre("visit_enter_function_scope",       //
+                                      "visit_enter_function_scope_body",  //
+                                      "visit_exit_function_scope",        //
+                                      "visit_end_of_module"));
+    EXPECT_THAT(v.errors,
+                ElementsAre(ERROR_TYPE_FIELD(
+                    error_missing_arrow_operator_in_arrow_function, where,
+                    offsets_matcher(&code, strlen(u8"(() "), u8"{"))));
+  }
+
+  {
+    padded_string code(u8"(()\n{});"_sv);
+    spy_visitor v;
+    parser p(&code, &v);
+    p.parse_and_visit_module(v);
+    EXPECT_THAT(v.visits, ElementsAre("visit_enter_function_scope",       //
+                                      "visit_enter_function_scope_body",  //
+                                      "visit_exit_function_scope",        //
+                                      "visit_end_of_module"));
+    EXPECT_THAT(v.errors,
+                ElementsAre(ERROR_TYPE_FIELD(
+                    error_missing_arrow_operator_in_arrow_function, where,
+                    offsets_matcher(&code, strlen(u8"(()\n"), u8"{"))));
+  }
+}
+
 TEST(test_parse, generator_function_with_misplaced_star) {
   {
     padded_string code(u8"function f*(x) { yield x; }"_sv);