Add support for single value row conditions (#5072)

crates/nu-parser/src/parser.rs

@@ -3993,7 +3993,7 @@ pub fn parse_math_expression(
     let mut last_prec = 1000000;
 
     let mut error = None;
-    let (lhs, err) = parse_value(
+    let (mut lhs, err) = parse_value(
         working_set,
         spans[0],
         &SyntaxShape::Any,
@@ -4002,6 +4002,13 @@ pub fn parse_math_expression(
     error = error.or(err);
     idx += 1;
 
+    if idx >= spans.len() {
+        // We already found the one part of our expression, so let's expand
+        if let Some(row_var_id) = lhs_row_var_id {
+            expand_to_cell_path(working_set, &mut lhs, row_var_id, expand_aliases_denylist);
+        }
+    }
+
     expr_stack.push(lhs);
 
     while idx < spans.len() {

src/tests/test_parser.rs

@@ -367,3 +367,11 @@ fn proper_rest_types() -> TestResult {
         "not verbose!",
     )
 }
+
+#[test]
+fn single_value_row_condition() -> TestResult {
+    run_test(
+        r#"[[a, b]; [true, false], [true, true]] | where a | length"#,
+        "2",
+    )
+}