fix: unwrap identity Date cast in comparison unwrapping

`unwrap_cast_in_comparison` refused to fold an identity `CAST(col AS DATE)`
on a `Date32` column, leaving a residual `Cast(col AS Date32)` in the
predicate instead of comparing against the bare column. SQL `cast(col AS
date)` plants a real `Expr::Cast` with no identity elision (unlike the
`arrow_cast` UDF, whose `simplify()` short-circuits identity), so this cast
reached `try_cast_literal_to_type` and was rejected.

Root cause is in `is_lossy_temporal_cast`:

    (is_date_type(from) && to.is_temporal()) || (is_date_type(to) && from.is_temporal())

For an identity `Date32 -> Date32` cast this is `true && true`, because
arrow's `DataType::is_temporal()` is true for `Date32`/`Date64`. The cast is
wrongly classified as a lossy temporal cast, `try_cast_literal_to_type`
returns `None`, and the residual cast is left in place.

Fix: short-circuit an identity cast (`from_type == to_type`) as non-lossy at
the top of `is_lossy_temporal_cast`. An identity cast can never change
comparison semantics.

This is deliberately scoped to *identical* types only. `Date32` counts days
and `Date64` counts milliseconds, but `try_cast_numeric_literal` uses
`mul = 1` for both, so a `Date32 <-> Date64` cast would convert units
wrongly and must stay blocked. A regression test asserts that.

Tests: identity `Date32 -> Date32` / `Date64 -> Date64` now fold;
`Date32 <-> Date64` still does not; plus an end-to-end simplifier test that
`cast(date_col AS DATE) = DATE '...'` simplifies to `date_col = <lit>`.

apache/datafusion `main` carries the same `is_lossy_temporal_cast`, so this
is also a candidate to upstream.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: adriangb

datafusion/expr-common/src/casts.rs

@@ -74,7 +74,19 @@ fn is_date_type(data_type: &DataType) -> bool {
 /// For example, `CAST(ts AS DATE) = DATE '2024-01-01'` means "any timestamp
 /// during that day", but unwrapping it to `ts = TIMESTAMP '2024-01-01
 /// 00:00:00'` matches only midnight.
+///
+/// An identity cast (`from_type == to_type`, e.g. `Date32 -> Date32`) never
+/// changes comparison semantics and is therefore not lossy. This has to be
+/// handled explicitly because `DataType::is_temporal()` is true for both
+/// `Date32` and `Date64`, so `is_date_type(from) && to.is_temporal()` would
+/// otherwise report an identity `Date -> Date` cast as lossy and block the
+/// rewrite. Note this is deliberately limited to *identical* types: a genuine
+/// `Date32 <-> Date64` cast changes units (days vs milliseconds) and must
+/// still be treated as lossy here.
 fn is_lossy_temporal_cast(from_type: &DataType, to_type: &DataType) -> bool {
+    if from_type == to_type {
+        return false;
+    }
     (is_date_type(from_type) && to_type.is_temporal())
         || (is_date_type(to_type) && from_type.is_temporal())
 }
@@ -760,6 +772,57 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_try_cast_identity_date_allowed() {
+        // An identity Date cast (e.g. `CAST(date_col AS DATE)` where the column
+        // is already Date32) must fold: it never changes comparison semantics,
+        // so `try_cast_literal_to_type` should return the same value rather than
+        // treating it as a lossy temporal cast.
+        expect_cast(
+            ScalarValue::Date32(Some(19_723)),
+            DataType::Date32,
+            ExpectedCast::Value(ScalarValue::Date32(Some(19_723))),
+        );
+
+        expect_cast(
+            ScalarValue::Date64(Some(1_704_067_200_000)),
+            DataType::Date64,
+            ExpectedCast::Value(ScalarValue::Date64(Some(1_704_067_200_000))),
+        );
+
+        // is_lossy_temporal_cast must classify an identity cast as non-lossy.
+        assert!(!is_lossy_temporal_cast(
+            &DataType::Date32,
+            &DataType::Date32
+        ));
+        assert!(!is_lossy_temporal_cast(
+            &DataType::Date64,
+            &DataType::Date64
+        ));
+    }
+
+    #[test]
+    fn test_try_cast_date32_date64_still_blocked() {
+        // `Date32` counts days and `Date64` counts milliseconds, but
+        // try_cast_numeric_literal uses mul = 1 for both, so a cross cast would
+        // convert units wrongly. The identity short-circuit must NOT open this
+        // up: Date32 <-> Date64 has to stay blocked.
+        assert!(is_lossy_temporal_cast(&DataType::Date32, &DataType::Date64));
+        assert!(is_lossy_temporal_cast(&DataType::Date64, &DataType::Date32));
+
+        expect_cast(
+            ScalarValue::Date32(Some(1)),
+            DataType::Date64,
+            ExpectedCast::NoValue,
+        );
+
+        expect_cast(
+            ScalarValue::Date64(Some(86_400_000)),
+            DataType::Date32,
+            ExpectedCast::NoValue,
+        );
+    }
+
     #[test]
     fn test_try_cast_to_type_unsupported() {
         // int64 to list

datafusion/optimizer/src/simplify_expressions/unwrap_cast.rs

@@ -586,6 +586,18 @@ mod tests {
         assert_eq!(optimize_test(expr_lt, &schema), expected);
     }
 
+    #[test]
+    /// An identity `CAST(date_col AS DATE)` on a Date32 column must fold away so
+    /// the comparison is against the bare column, e.g. `date_col = DATE '...'`.
+    fn test_unwrap_identity_date_cast() {
+        let schema = expr_test_schema();
+        // cast(date32_col as Date32) = Date32(19723)  ->  date32_col = Date32(19723)
+        let lit_date = lit(ScalarValue::Date32(Some(19_723)));
+        let expr_eq = cast(col("date32_col"), DataType::Date32).eq(lit_date.clone());
+        let expected = col("date32_col").eq(lit_date);
+        assert_eq!(optimize_test(expr_eq, &schema), expected);
+    }
+
     fn optimize_test(expr: Expr, schema: &DFSchemaRef) -> Expr {
         let simplifier = ExprSimplifier::new(
             SimplifyContext::builder()
@@ -608,6 +620,7 @@ mod tests {
                     Field::new("c6", DataType::UInt32, false),
                     Field::new("ts_nano_none", timestamp_nano_none_type(), false),
                     Field::new("ts_nano_utf", timestamp_nano_utc_type(), false),
+                    Field::new("date32_col", DataType::Date32, false),
                     Field::new("str1", DataType::Utf8, false),
                     Field::new("largestr", DataType::LargeUtf8, false),
                     Field::new("tag", dictionary_tag_type(), false),