From cd1994b63e32191640cca80da4fd420af0650378 Mon Sep 17 00:00:00 2001
From: Ritchie Vink <ritchie46@gmail.com>
Date: Sun, 31 Mar 2024 10:39:20 +0200
Subject: [PATCH] fix: Don't prune alias in function subtree (#15406)

---
 crates/polars-plan/src/logical_plan/conversion.rs | 8 +-------
 py-polars/tests/unit/test_schema.py               | 5 +++++
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/crates/polars-plan/src/logical_plan/conversion.rs b/crates/polars-plan/src/logical_plan/conversion.rs
index 3c1f6f65ab3a..c78e8c5b56ea 100644
--- a/crates/polars-plan/src/logical_plan/conversion.rs
+++ b/crates/polars-plan/src/logical_plan/conversion.rs
@@ -221,13 +221,7 @@ fn to_aexpr_impl(expr: Expr, arena: &mut Arena<AExpr>, state: &mut ConversionSta
             function,
             options,
         } => {
-            match function {
-                #[cfg(feature = "dtype-struct")]
-                FunctionExpr::AsStruct => {
-                    state.prune_alias = false;
-                },
-                _ => {},
-            }
+            state.prune_alias = false;
             AExpr::Function {
                 input: to_aexprs(input, arena, state),
                 function,
diff --git a/py-polars/tests/unit/test_schema.py b/py-polars/tests/unit/test_schema.py
index 1950486799a7..41d8c6777aef 100644
--- a/py-polars/tests/unit/test_schema.py
+++ b/py-polars/tests/unit/test_schema.py
@@ -638,3 +638,8 @@ def test_literal_subtract_schema_13284() -> None:
 def test_schema_boolean_sum_horizontal() -> None:
     lf = pl.LazyFrame({"a": [True, False]}).select(pl.sum_horizontal("a"))
     assert lf.schema == OrderedDict([("a", pl.UInt32)])
+
+
+def test_struct_alias_prune_15401() -> None:
+    df = pl.DataFrame({"a": []}, schema={"a": pl.Struct({"b": pl.Int8})})
+    assert df.select(pl.col("a").alias("c").struct.field("b")).columns == ["b"]