Pandas BUG: In main, TimedeltaIndex.shift() requires freq in the index, but it may not be available because it was computed

Pandas version checks

[x] I have checked that this issue has not already been reported.
[x] I have confirmed this bug exists on the latest version of pandas.
[x] I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import pandas as pd
ind = pd.date_range("1/1/2021", "1/5/2021") - pd.Timestamp("1/3/2019")
ind.shift(1)

Issue Description

This only occurs on main. NOT a current bug in pandas.

Gives error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Code\pandas_dev\pandas\pandas\core\indexes\datetimelike.py", line 512, in shift
    raise NullFrequencyError("Cannot shift with no freq")
pandas.errors.NullFrequencyError: Cannot shift with no freq

The above code works fine with pandas 2.3.

Expected Behavior

No error. A user doing a calculation that produces a TimedeltaIndex can't be expected to set the freq of the index.

I think was introduced by @jbrockmendel in #61985

Installed Versions

INSTALLED VERSIONS ------------------ commit : bb10b27dea9d9a2476de4c8122e0346689e1c9c3 python : 3.11.13 python-bits : 64 OS : Windows OS-release : 10 Version : 10.0.26100 machine : AMD64 processor : Intel64 Family 6 Model 183 Stepping 1, GenuineIntel byteorder : little LC_ALL : None LANG : None LOCALE : English_United States.1252 pandas : 3.0.0.dev0+2306.gbb10b27dea.dirty numpy : 2.2.6 dateutil : 2.9.0.post0 pip : 25.2 Cython : 3.1.2 sphinx : 8.2.3 IPython : 9.4.0 adbc-driver-postgresql: None adbc-driver-sqlite : None bs4 : 4.13.4 bottleneck : 1.5.0 fastparquet : 2024.11.0 fsspec : 2025.7.0 html5lib : 1.1 hypothesis : 6.137.3 gcsfs : 2025.7.0 jinja2 : 3.1.6 lxml.etree : 6.0.0 matplotlib : 3.10.5 numba : 0.61.2 numexpr : 2.10.2 odfpy : None openpyxl : 3.1.5 psycopg2 : 2.9.10 pymysql : 1.4.6 pyarrow : 19.0.1 pyiceberg : 0.9.1 pyreadstat : 1.3.0 pytest : 8.4.1 python-calamine : None pytz : 2025.2 pyxlsb : 1.0.10 s3fs : 2025.7.0 scipy : 1.16.1 sqlalchemy : 2.0.43 tables : 3.10.2 tabulate : 0.9.0 xarray : 2025.7.1 xlrd : 2.0.1 xlsxwriter : 3.2.5 zstandard : 0.23.0 qtpy : None pyqt5 : None

Comment From: jbrockmendel

Should be straightforward to add this case to _get_arithmetic_result_freq. Marking as Good First Issue.

Comment From: Divyanshu-spec-code

Hi, I’m new to open source and would like to work on this issue. Could you please assign it to me?

Comment From: GouravPriyadarshi

Hello all, This is the first time I am attempting for open source contribution. Any suggestions or how can I move forward on this issue?