Attempts to fix issue #266

src/quick-lint-js/error.h

@@ -214,6 +214,13 @@
       .error(QLJS_TRANSLATABLE("'else' has no corresponding 'if'"),            \
              else_token))                                                      \
                                                                                \
+  QLJS_ERROR_TYPE(                                                             \
+      error_else_with_conditional_missing_if, "E???",                          \
+      { source_code_span else_token; },                                        \
+      .warning(QLJS_TRANSLATABLE("'else' with condition followed by block; "   \
+                                 "maybe 'else if' was intended"),              \
+               else_token))                                                    \
+                                                                               \
   QLJS_ERROR_TYPE(                                                             \
       error_escaped_character_disallowed_in_identifiers, "E012",               \
       { source_code_span escape_sequence; },                                   \

src/quick-lint-js/parse.h

@@ -2515,8 +2515,26 @@ class parser {
     }
 
     if (this->peek().type == token_type::kw_else) {
+      source_code_span else_span = this->peek().span();
       this->skip();
-      parse_and_visit_body();
+
+      switch (this->peek().type) {
+      default:
+        parse_and_visit_body();
+        break;
+
+      case token_type::left_paren:
+        expression *ast = this->parse_expression(precedence{});
+        this->visit_expression(ast, v, variable_context::rhs);
+
+        if (this->peek().type == token_type::left_curly) {
+          this->error_reporter_->report(error_else_with_conditional_missing_if{
+              .else_token = else_span,
+          });
+          parse_and_visit_body();
+        }
+        break;
+      }
     }
   }
 

test/test-parse-statement.cpp

@@ -578,6 +578,42 @@ TEST(test_parse, else_without_if) {
   }
 }
 
+TEST(test_parse, else_if) {
+  {
+    spy_visitor v =
+        parse_and_visit_statement(u8"if (a) { b; } else if (c) { d; }"_sv);
+    EXPECT_THAT(v.visits, ElementsAre("visit_variable_use",        // a
+                                      "visit_enter_block_scope",   // <if>
+                                      "visit_variable_use",        // b
+                                      "visit_exit_block_scope",    // </if>
+                                      "visit_variable_use",        // c
+                                      "visit_enter_block_scope",   // <else>
+                                      "visit_variable_use",        // d
+                                      "visit_exit_block_scope"));  // </else>
+  }
+
+  {
+    spy_visitor v;
+    padded_string code(u8"if (a) { b; } else (c) { d; }"_sv);
+    parser p(&code, &v);
+    EXPECT_TRUE(p.parse_and_visit_statement(v));
+    EXPECT_THAT(v.visits, ElementsAre("visit_variable_use",        // a
+                                      "visit_enter_block_scope",   // <if>
+                                      "visit_variable_use",        // b
+                                      "visit_exit_block_scope",    // </if>
+                                      "visit_variable_use",        // c
+                                      "visit_enter_block_scope",   // <block>
+                                      "visit_variable_use",        // d
+                                      "visit_exit_block_scope"));  // </block>
+
+    EXPECT_THAT(
+        v.errors,
+        ElementsAre(ERROR_TYPE_FIELD(
+            error_else_with_conditional_missing_if, else_token,
+            offsets_matcher(&code, strlen(u8"if (a) { b; } "), u8"else"))));
+  }
+}
+
 TEST(test_parse, block_statement) {
   {
     spy_visitor v = parse_and_visit_statement(u8"{ }"_sv);