Use explicit cast to address type errors

Author: spencerkclark

xarray/core/dtypes.py

@@ -10,6 +10,7 @@
 from xarray.compat import array_api_compat, npcompat
 from xarray.compat.npcompat import HAS_STRING_DTYPE
 from xarray.core import utils
+from xarray.core.types import PDDatetimeUnitOptions
 
 if TYPE_CHECKING:
     from typing import Any
@@ -89,7 +90,10 @@ def maybe_promote(dtype: T_dtype) -> tuple[T_dtype, Any]:
         # np.timedelta64 is a subclass of np.integer
         # Check np.timedelta64 before np.integer
         unit, _ = np.datetime_data(dtype)
-        fill_value = np.timedelta64("NaT", unit)  # type: ignore[call-overload]
+        # np.datetime_data returns a generic str so we need to cast it to
+        # a valid time unit for mypy purposes.
+        unit = cast(PDDatetimeUnitOptions, unit)
+        fill_value = np.timedelta64("NaT", unit)
         dtype_ = dtype
     elif isdtype(dtype, "integral"):
         dtype_ = np.float32 if dtype.itemsize <= 2 else np.float64
@@ -99,8 +103,11 @@ def maybe_promote(dtype: T_dtype) -> tuple[T_dtype, Any]:
         fill_value = np.nan + np.nan * 1j
     elif np.issubdtype(dtype, np.datetime64):
         unit, _ = np.datetime_data(dtype)
+        # np.datetime_data returns a generic str so we need to cast it to
+        # a valid time unit for mypy purposes.
+        unit = cast(PDDatetimeUnitOptions, unit)
         dtype_ = dtype
-        fill_value = np.datetime64("NaT", unit)  # type: ignore[call-overload]
+        fill_value = np.datetime64("NaT", unit)
     else:
         dtype_ = object
         fill_value = np.nan