compare expand_cols by variant not exact datatype (#3050)

polars/polars-lazy/src/logical_plan/projection.rs

@@ -145,7 +145,12 @@ fn expand_columns(expr: &Expr, result: &mut Vec<Expr>, names: &[String]) {
 /// replace `DtypeColumn` with `col("foo")..col("bar")`
 fn expand_dtypes(expr: &Expr, result: &mut Vec<Expr>, schema: &Schema, dtypes: &[DataType]) {
     for dtype in dtypes {
-        for field in schema.iter_fields().filter(|f| f.data_type() == dtype) {
+        // we compare by variant not by exact datatype as units/ refmaps etc may differ.
+        let variant = std::mem::discriminant(dtype);
+        for field in schema
+            .iter_fields()
+            .filter(|f| std::mem::discriminant(f.data_type()) == variant)
+        {
             let name = field.name();
 
             let mut new_expr = expr.clone();

polars/tests/it/lazy/expressions/expand.rs

@@ -0,0 +1,39 @@
+use super::*;
+use polars::export::chrono::{NaiveDate, NaiveDateTime};
+
+#[test]
+fn test_expand_datetimes_3042() -> Result<()> {
+    let low = NaiveDate::from_ymd(2020, 1, 1).and_hms(0, 0, 0);
+    let high = NaiveDate::from_ymd(2020, 2, 1).and_hms(0, 0, 0);
+    let date_range = date_range(
+        "dt1",
+        low,
+        high,
+        Duration::parse("1w"),
+        ClosedWindow::Left,
+        TimeUnit::Milliseconds,
+    )
+    .into_series();
+
+    let out = df![
+        "dt1" => date_range.clone(),
+        "dt2" => date_range,
+    ]?
+    .lazy()
+    // this tests if we expand datetimes even though the units differ
+    .with_column(
+        dtype_col(&DataType::Datetime(TimeUnit::Microseconds, None))
+            .dt()
+            .strftime("%m/%d/%Y"),
+    )
+    .limit(3)
+    .collect()?;
+
+    let expected = df![
+        "dt1" => ["01/01/2020", "01/08/2020", "01/15/2020"],
+        "dt2" => ["01/01/2020", "01/08/2020", "01/15/2020"],
+    ]?;
+    assert!(out.frame_equal(&expected));
+
+    Ok(())
+}

polars/tests/it/lazy/expressions/mod.rs

@@ -1,5 +1,6 @@
 mod apply;
 mod arity;
+mod expand;
 mod is_in;
 mod slice;
 mod window;