Validate APPEND custom commands more strictly

Author: mrexodia

docs/cmake-toml.md

@@ -335,7 +335,7 @@ command-expand-lists = false
 uses-terminal = false
 ```
 
-For each custom command entry, exactly one of `outputs` or `build-event` is required. Relative `outputs`/`byproducts` paths follow CMake and are interpreted from the current binary directory. `build-event` is only valid for non-`interface` targets, and `pre-link` is not supported for `type = "custom"`. In both `add_custom_command(OUTPUT ...)` and `add_custom_target(...)`, `job-pool` cannot be combined with `uses-terminal = true`.
+For each custom command entry, exactly one of `outputs` or `build-event` is required. Relative `outputs`/`byproducts` paths follow CMake and are interpreted from the current binary directory. `build-event` is only valid for non-`interface` targets, and `pre-link` is not supported for `type = "custom"`. In both `add_custom_command(OUTPUT ...)` and `add_custom_target(...)`, `job-pool` cannot be combined with `uses-terminal = true`. When `append = true` is used with the `OUTPUT` form, at least one `command`/`commands` entry or `depends` must also be provided.
 
 A table mapping the cmkr features to the relevant CMake construct and the relevant documentation pages:
 

src/project_parser.cpp

@@ -1029,7 +1029,11 @@ Project::Project(const Project *parent, const std::string &path, bool build) : p
                     }
                 }
 
-                if (custom_command.commands.empty() && !(custom_command.is_output_form() && custom_command.append)) {
+                if (custom_command.is_output_form() && custom_command.append && custom_command.commands.empty() && custom_command.depends.empty()) {
+                    throw_key_error("append requires at least one command or depends", "append", custom.find("append"));
+                }
+
+                if (custom_command.commands.empty() && !(custom_command.is_output_form() && custom_command.append && !custom_command.depends.empty())) {
                     throw_key_error("At least one command is required", "custom-command", custom_command_value);
                 }