give a hard error upon redefining a built-in command

previously, aliases to built-in commands were silently ignored. this matches git's behavior, but seems unhelpful, especially if the user doesn't know that a command with that name already exists.
give a hard error rather than silently ignoring it.

Author: jyn514

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 * The minimum supported Rust version (MSRV) is now 1.76.0.
 
+* Attempting to alias a built-in command now gives a hard error, rather than being silently ignored.
+
 ### New features
 
 * Templates now support logical operators: `||`, `&&`, `!`

cli/src/cli_util.rs

@@ -2699,6 +2699,7 @@ fn resolve_aliases(
             }
         }
     }
+
     let mut resolved_aliases = HashSet::new();
     let mut real_commands = HashSet::new();
     for command in app.get_subcommands() {
@@ -2707,6 +2708,14 @@ fn resolve_aliases(
             real_commands.insert(alias.to_string());
         }
     }
+    for alias in aliases_map.keys() {
+        if real_commands.contains(alias) {
+            return Err(user_error(format!(
+                "Cannot define an alias that overrides the built-in command '{alias}'"
+            )));
+        }
+    }
+
     loop {
         let app_clone = app.clone().allow_external_subcommands(true);
         let matches = app_clone.try_get_matches_from(&string_args).ok();

cli/tests/test_alias.rs

@@ -164,10 +164,10 @@ fn test_alias_cannot_override_builtin() {
     let repo_path = test_env.env_root().join("repo");
 
     test_env.add_config(r#"aliases.log = ["rebase"]"#);
-    // Alias should be ignored
-    let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-r", "root()"]);
-    insta::assert_snapshot!(stdout, @r###"
-    ◉  zzzzzzzz root() 00000000
+    // Alias should give a hard error
+    let stderr = test_env.jj_cmd_failure(&repo_path, &["log", "-r", "root()"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Error: Cannot define an alias that overrides the built-in command 'log'
     "###);
 }