From 437a2d60e45012fb079ec5d06cd3e9299b8735f2 Mon Sep 17 00:00:00 2001
From: Matthew Murray <matthewmurray711@gmail.com>
Date: Wed, 19 Aug 2026 14:21:57 +0000
Subject: [PATCH] fix(to_numpy): return object dtype for bool/numeric and
 datetime/timedelta mixes in pandas-compatible mode

---
 python/cudf/cudf/core/frame.py   | 19 ++++++++++++++++---
 python/cudf/cudf/utils/dtypes.py | 30 ++++++++++++++++++------------
 2 files changed, 34 insertions(+), 15 deletions(-)

diff --git a/python/cudf/cudf/core/frame.py b/python/cudf/cudf/core/frame.py
index 68aa59d65e22..db400046aeaa 100644
--- a/python/cudf/cudf/core/frame.py
+++ b/python/cudf/cudf/core/frame.py
@@ -35,6 +35,7 @@
 )
 from cudf.core.mixins import BinaryOperand, Scannable
 from cudf.utils.dtypes import (
+    _is_pandas_object_dtype_mix,
     dtype_from_pylibcudf_column,
     find_common_type,
     is_pandas_nullable_extension_dtype,
@@ -719,9 +720,21 @@ def to_array(
                     # otherwise store the null sentinel instead.
                     to_dtype = np.dtype("float64")
             else:
-                to_dtype = find_common_type(
-                    [dtype for _, dtype in self._dtypes]
-                )
+                column_dtypes = [dtype for _, dtype in self._dtypes]
+                if (
+                    module is not cupy
+                    and cudf.get_option("mode.pandas_compatible")
+                    and _is_pandas_object_dtype_mix(column_dtypes)
+                ):
+                    # pandas coerces bool+numeric and datetime64+timedelta64
+                    # mixes to `object` rather than promoting them; match
+                    # that here instead of calling find_common_type (which
+                    # raises in this case so the cudf.pandas proxy can fall
+                    # back to pandas). GPU arrays can't hold objects, so this
+                    # fallback only applies to numpy output.
+                    to_dtype = np.dtype("object")
+                else:
+                    to_dtype = find_common_type(column_dtypes)
                 if to_dtype is not None and any(
                     col.has_nulls() for col in self._columns
                 ):
diff --git a/python/cudf/cudf/utils/dtypes.py b/python/cudf/cudf/utils/dtypes.py
index 8322926040f0..9d715d94f7a7 100644
--- a/python/cudf/cudf/utils/dtypes.py
+++ b/python/cudf/cudf/utils/dtypes.py
@@ -304,6 +304,18 @@ def _get_nan_for_dtype(dtype: DtypeObj) -> ScalarLike:
         return np.float64("nan")
 
 
+def _is_pandas_object_dtype_mix(dtypes: Iterable[DtypeObj]) -> bool:
+    """
+    Whether ``dtypes`` mixes bool with numeric, or datetime64 with
+    timedelta64. Pandas coerces these combinations to ``object`` rather
+    than promoting via NumPy's type promotion rules.
+    """
+    kinds = {dtype.kind for dtype in dtypes if isinstance(dtype, np.dtype)}
+    return ("b" in kinds and bool(kinds & set("iuf"))) or (
+        "M" in kinds and "m" in kinds
+    )
+
+
 def find_common_type(dtypes: Iterable[DtypeObj]) -> DtypeObj:
     """
     Wrapper over np.result_type to handle cudf specific types.
@@ -398,24 +410,18 @@ def find_common_type(dtypes: Iterable[DtypeObj]) -> DtypeObj:
             "not supported"
         )
 
-    if pandas_compatible:
+    if pandas_compatible and _is_pandas_object_dtype_mix(dtypes):
         # cudf follows NumPy promotion: bool+int->int, bool+float->float,
         # datetime64+timedelta64->datetime64. Pandas returns `object` for
         # these mixes. Raise so that, when used as the cudf.pandas fast path
         # for `pandas.core.dtypes.cast.find_common_type`, we fall back to
         # pandas' implementation rather than silently producing a different
         # answer.
-        kinds = {dtype.kind for dtype in dtypes if isinstance(dtype, np.dtype)}
-        if "b" in kinds and kinds & set("iuf"):
-            raise NotImplementedError(
-                "Common type of bool with numeric dtypes is not supported "
-                "in pandas-compatible mode."
-            )
-        if "M" in kinds and "m" in kinds:
-            raise NotImplementedError(
-                "Common type of datetime64 with timedelta64 is not supported "
-                "in pandas-compatible mode."
-            )
+        raise NotImplementedError(
+            "Common type of bool with numeric, or datetime64 with "
+            "timedelta64, dtypes is not supported in pandas-compatible "
+            "mode."
+        )
 
     try:
         common_dtype = np.result_type(*dtypes)  # noqa: TID251