BUG: ensure_timedelta64ns overflows

pandas/_libs/tslibs/conversion.pyx

@@ -217,11 +217,35 @@ def ensure_timedelta64ns(arr: ndarray, copy: bool=True):
 
     Returns
     -------
-    result : ndarray with dtype timedelta64[ns]
-
+    ndarray[timedelta64[ns]]
     """
-    return arr.astype(TD64NS_DTYPE, copy=copy)
-    # TODO: check for overflows when going from a lower-resolution to nanos
+    assert arr.dtype.kind == "m", arr.dtype
+
+    if arr.dtype == TD64NS_DTYPE:
+        return arr.copy() if copy else arr
+
+    # Re-use the datetime64 machinery to do an overflow-safe `astype`
+    dtype = arr.dtype.str.replace("m8", "M8")
+    dummy = arr.view(dtype)
+    try:
+        dt64_result = ensure_datetime64ns(dummy, copy)
+    except OutOfBoundsDatetime as err:
+        # Re-write the exception in terms of timedelta64 instead of dt64
+
+        # Find the value that we are going to report as causing an overflow
+        tdmin = arr.min()
+        tdmax = arr.max()
+        if np.abs(tdmin) >= np.abs(tdmax):
+            bad_val = tdmin
+        else:
+            bad_val = tdmax
+
+        unit_str = arr.dtype.str.split("[")[-1][:-1]
+        raise OutOfBoundsDatetime(
+            f"Out of bounds for nanosecond timedelta {bad_val}[{unit_str}]"
+        )
+
+    return dt64_result.view(TD64NS_DTYPE)
 
 
 # ----------------------------------------------------------------------

pandas/core/internals/blocks.py

@@ -2321,7 +2321,8 @@ class TimeDeltaBlock(DatetimeLikeBlockMixin, IntBlock):
 
     def __init__(self, values, placement, ndim=None):
         if values.dtype != TD64NS_DTYPE:
-            values = conversion.ensure_timedelta64ns(values)
+            # e.g. non-nano or int64
+            values = TimedeltaArray._from_sequence(values)._data
         if isinstance(values, TimedeltaArray):
             values = values._data
         assert isinstance(values, np.ndarray), type(values)

pandas/tests/tslibs/test_conversion.py

@@ -4,7 +4,13 @@
 import pytest
 from pytz import UTC
 
-from pandas._libs.tslibs import conversion, iNaT, timezones, tzconversion
+from pandas._libs.tslibs import (
+    OutOfBoundsDatetime,
+    conversion,
+    iNaT,
+    timezones,
+    tzconversion,
+)
 
 from pandas import Timestamp, date_range
 import pandas._testing as tm
@@ -80,6 +86,13 @@ def test_ensure_datetime64ns_bigendian():
     tm.assert_numpy_array_equal(result, expected)
 
 
+def test_ensure_timedelta64ns_overflows():
+    arr = np.arange(10).astype("m8[Y]") * 100
+    msg = r"Out of bounds for nanosecond timedelta 900\[Y\]"
+    with pytest.raises(OutOfBoundsDatetime, match=msg):
+        conversion.ensure_timedelta64ns(arr)
+
+
 class SubDatetime(datetime):
     pass