fix: Fix PartialEq for DataType::Unknown

crates/polars-core/src/datatypes/dtype.rs

@@ -120,6 +120,10 @@ impl PartialEq for DataType {
                 (Array(left_inner, left_width), Array(right_inner, right_width)) => {
                     left_width == right_width && left_inner == right_inner
                 },
+                (Unknown(l), Unknown(r)) => match (l, r) {
+                    (UnknownKind::Int(_), UnknownKind::Int(_)) => true,
+                    _ => l == r,
+                },
                 _ => std::mem::discriminant(self) == std::mem::discriminant(other),
             }
         }

crates/polars-plan/src/dsl/function_expr/schema.rs

@@ -511,12 +511,9 @@ impl<'a> FieldsMapper<'a> {
             Some(dtype) => dtype,
             // Supertype of `new` and `default`
             None => {
-                let default = if let Some(default) = self.fields.get(3) {
-                    default
-                } else {
-                    &self.fields[0]
-                };
-                try_get_supertype(self.fields[2].data_type(), default.data_type())?
+                let column_type = &self.fields[0];
+                let new = &self.fields[2];
+                try_get_supertype(column_type.data_type(), new.data_type())?
             },
         };
         self.with_dtype(dtype)

crates/polars-plan/src/logical_plan/optimizer/type_coercion/binary.rs

@@ -240,15 +240,7 @@ pub(super) fn process_binary(
                 right: node_right,
             }));
         },
-        (Unknown(lhs), Unknown(rhs)) if lhs == rhs => {
-            // Materialize if both are dynamic
-            let left = unpack!(materialize(left));
-            let right = unpack!(materialize(right));
-            let left = expr_arena.add(left);
-            let right = expr_arena.add(right);
-
-            return Ok(Some(AExpr::BinaryExpr { left, op, right }));
-        },
+        (Unknown(lhs), Unknown(rhs)) if lhs == rhs => return Ok(None),
         _ => {
             unpack!(early_escape(&type_left, &type_right));
         },
@@ -328,7 +320,6 @@ pub(super) fn process_binary(
         Ok(None)
     } else {
         // Coerce types:
-
         let st = unpack!(get_supertype(&type_left, &type_right));
         let mut st = modify_supertype(st, left, right, &type_left, &type_right);
 

py-polars/tests/unit/functions/test_when_then.py

@@ -596,3 +596,11 @@ def test_when_then_parametric(
             assert_frame_equal(ref, ans)
         else:
             assert ref["if_true"].to_list() == ans["if_true"].to_list()
+
+
+def test_when_then_supertype_15975() -> None:
+    df = pl.DataFrame({"a": [1, 2, 3]})
+
+    assert df.with_columns(
+        pl.when(True).then(1 ** pl.col("a") + 1.0 * pl.col("a"))
+    ).to_dict(as_series=False) == {"a": [1, 2, 3], "literal": [2.0, 3.0, 4.0]}

py-polars/tests/unit/operations/test_replace.py

@@ -472,14 +472,14 @@ def test_replace_fast_path_many_to_one() -> None:
 def test_replace_fast_path_many_to_one_default() -> None:
     lf = pl.LazyFrame({"a": [1, 2, 2, 3]})
     result = lf.select(pl.col("a").replace([2, 3], 100, default=-1))
-    expected = pl.LazyFrame({"a": [-1, 100, 100, 100]}, schema={"a": pl.Int32})
+    expected = pl.LazyFrame({"a": [-1, 100, 100, 100]}, schema={"a": pl.Int64})
     assert_frame_equal(result, expected)
 
 
 def test_replace_fast_path_many_to_one_null() -> None:
     lf = pl.LazyFrame({"a": [1, 2, 2, 3]})
     result = lf.select(pl.col("a").replace([2, 3], None, default=-1))
-    expected = pl.LazyFrame({"a": [-1, None, None, None]}, schema={"a": pl.Int32})
+    expected = pl.LazyFrame({"a": [-1, None, None, None]}, schema={"a": pl.Int64})
     assert_frame_equal(result, expected)