BUG: RangeIndex.union not sorting results when sort=True in pandas-2.0

[galipremsagar]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

>>> idx1 = pd.RangeIndex(1, 100, 6)
>>> idx2 = pd.RangeIndex(1, 50, 3)
>>> idx1.union(idx2, sort=True)
Index([ 1,  7, 13, 19, 25, 31, 37, 43, 49, 55, 61, 67, 73, 79, 85, 91, 97,  4,
       10, 16, 22, 28, 34, 40, 46],
      dtype='int64')
>>> sort
True
>>> idx1.union(idx2, sort=None)
Index([ 1,  4,  7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 55,
       61, 67, 73, 79, 85, 91, 97],
      dtype='int64')

Issue Description

When sort=True, RangeIndex.union doesn't seem to sort results. But does sort when sort=None.

Expected Behavior

>>> idx1.union(idx2, sort=True)
Index([ 1,  4,  7, 10, 13, 16, 19, 22, 25, 28, 31, 34, 37, 40, 43, 46, 49, 55,
       61, 67, 73, 79, 85, 91, 97],
      dtype='int64')

Installed Versions

INSTALLED VERSIONS

commit : 965ceca
python : 3.10.11.final.0
python-bits : 64
OS : Linux
OS-release : 4.15.0-76-generic
Version : #86-Ubuntu SMP Fri Jan 17 17:24:28 UTC 2020
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 2.0.2
numpy : 1.24.3
pytz : 2023.3
dateutil : 2.8.2
setuptools : 67.7.2
pip : 23.1.2
Cython : 0.29.35
pytest : 7.3.1
hypothesis : 6.75.7
sphinx : 5.3.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.13.2
pandas_datareader: None
bs4 : 4.12.2
bottleneck : None
brotli :
fastparquet : None
fsspec : 2023.5.0
gcsfs : None
matplotlib : None
numba : 0.57.0
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 11.0.0
pyreadstat : None
pyxlsb : None
s3fs : 2023.5.0
scipy : 1.10.1
snappy :
sqlalchemy : 2.0.15
tables : None
tabulate : 0.9.0
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[mroeschke]

Thanks could you provide idx1 and idx2?

[galipremsagar]

@mroeschke Apologies for missing out the full repro. Updated the example above.