Fix defaulting alias var values (#2631)

crates/nu-cli/src/commands/run_alias.rs

@@ -4,7 +4,7 @@ use crate::prelude::*;
 
 use derive_new::new;
 use nu_errors::ShellError;
-use nu_protocol::{hir::Block, Scope, Signature, SyntaxShape};
+use nu_protocol::{hir::Block, Scope, Signature, SyntaxShape, UntaggedValue};
 
 #[derive(new, Clone)]
 pub struct AliasCommand {
@@ -51,12 +51,24 @@ impl WholeStreamCommand for AliasCommand {
         let evaluated = call_info.evaluate(&registry).await?;
 
         let mut vars = IndexMap::new();
+
+        let mut num_positionals = 0;
         if let Some(positional) = &evaluated.args.positional {
+            num_positionals = positional.len();
             for (pos, arg) in positional.iter().enumerate() {
                 vars.insert(alias_command.args[pos].0.to_string(), arg.clone());
             }
         }
 
+        if alias_command.args.len() > num_positionals {
+            for idx in 0..(alias_command.args.len() - num_positionals) {
+                vars.insert(
+                    alias_command.args[idx + num_positionals].0.to_string(),
+                    UntaggedValue::nothing().into_untagged_value(),
+                );
+            }
+        }
+
         let scope = Scope::append_vars(scope, vars);
 
         // FIXME: we need to patch up the spans to point at the top-level error

crates/nu-cli/tests/commands/alias.rs

@@ -16,6 +16,21 @@ fn alias_args_work() {
     })
 }
 
+#[test]
+fn alias_missing_args_work() {
+    Playground::setup("append_test_1", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.root(),
+            r#"
+                alias double_echo [a b] {^echo $a $b}
+                double_echo bob
+            "#
+        );
+
+        assert_eq!(actual.out, "bob");
+    })
+}
+
 #[test]
 #[cfg(not(windows))]
 fn alias_parses_path_tilde() {