fix: Struct filter by index (#18778)

crates/polars-plan/src/plans/conversion/dsl_to_ir.rs

@@ -811,6 +811,11 @@ fn expand_filter(
         | Expr::DtypeColumn(_)
         | Expr::IndexColumn(_)
         | Expr::Nth(_) => true,
+        #[cfg(feature = "dtype-struct")]
+        Expr::Function {
+            function: FunctionExpr::StructExpr(StructFunction::FieldByIndex(_)),
+            ..
+        } => true,
         _ => false,
     }) {
         let mut rewritten = rewrite_projections(vec![predicate], &schema, &[], opt_flags)?;

py-polars/tests/unit/operations/namespaces/test_struct.py

@@ -3,7 +3,12 @@
 import datetime
 from collections import OrderedDict
 
+import pytest
+
 import polars as pl
+from polars.exceptions import (
+    OutOfBoundsError,
+)
 from polars.testing import assert_frame_equal
 
 
@@ -95,3 +100,20 @@ def test_empty_list_eval_schema_5734() -> None:
     assert df.filter(False).select(
         pl.col("a").list.eval(pl.element().struct.field("b"))
     ).schema == {"a": pl.List(pl.Int64)}
+
+
+def test_field_by_index_18732() -> None:
+    df = pl.DataFrame({"foo": [{"a": 1, "b": 2}, {"a": 2, "b": 1}]})
+
+    # illegal upper bound
+    with pytest.raises(OutOfBoundsError, match=r"index 2 for length: 2"):
+        df.filter(pl.col.foo.struct[2] == 1)
+
+    # legal
+    expected_df = pl.DataFrame({"foo": [{"a": 1, "b": 2}]})
+    result_df = df.filter(pl.col.foo.struct[0] == 1)
+    assert_frame_equal(expected_df, result_df)
+
+    expected_df = pl.DataFrame({"foo": [{"a": 2, "b": 1}]})
+    result_df = df.filter(pl.col.foo.struct[-1] == 1)
+    assert_frame_equal(expected_df, result_df)