From 0b040a1e5ac498d36e01bf5cb00d1b089aafa7d4 Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Mon, 30 Oct 2023 08:00:20 -0400
Subject: [PATCH 1/4] BUG: DatetimeIndex.diff raising TypeError

---
 pandas/core/indexes/base.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 761f5df3bb4e0..a1bfe11becdad 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -7017,7 +7017,11 @@ def diff(self, periods: int = 1) -> Self:
         Index([nan, 10.0, 10.0, 10.0, 10.0], dtype='float64')
 
         """
-        return self._constructor(self.to_series().diff(periods))
+        diff = self.to_series().diff(periods)
+        if diff.dtype != self.dtype:
+            # e.g. DTI.diff -> TDI
+            return Index(diff)
+        return self._constructor(diff)
 
     @final
     def round(self, decimals: int = 0) -> Self:

From 6dcefdcab5a0b31ec59c3b138bf6370d305f2a2a Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Mon, 30 Oct 2023 08:01:59 -0400
Subject: [PATCH 2/4] add test and whatsnew

---
 doc/source/whatsnew/v2.1.3.rst            | 2 +-
 pandas/tests/indexes/test_datetimelike.py | 8 ++++++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v2.1.3.rst b/doc/source/whatsnew/v2.1.3.rst
index 1359123ef153e..ec5103c09215a 100644
--- a/doc/source/whatsnew/v2.1.3.rst
+++ b/doc/source/whatsnew/v2.1.3.rst
@@ -13,7 +13,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
--
+- Fixed regression in :meth:`DatetimeIndex.diff` raising ``TypeError`` (:issue:`55080`)
 -
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/tests/indexes/test_datetimelike.py b/pandas/tests/indexes/test_datetimelike.py
index 71cc7f29c62bc..5ad2e9b2f717e 100644
--- a/pandas/tests/indexes/test_datetimelike.py
+++ b/pandas/tests/indexes/test_datetimelike.py
@@ -159,3 +159,11 @@ def test_where_cast_str(self, simple_index):
 
         result = index.where(mask, ["foo"])
         tm.assert_index_equal(result, expected)
+
+    @pytest.mark.parametrize("unit", ["ns", "us", "ms", "s"])
+    def test_diff(self, unit):
+        # GH 55080
+        dti = pd.to_datetime([10, 20, 30], unit=unit).as_unit(unit)
+        result = dti.diff(1)
+        expected = pd.TimedeltaIndex([pd.NaT, 10, 10], unit=unit).as_unit(unit)
+        tm.assert_index_equal(result, expected)

From 0894b0c8aee2bdc3ba6990f44ee214c1ed19835f Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Mon, 30 Oct 2023 08:05:31 -0400
Subject: [PATCH 3/4] typing

---
 pandas/core/indexes/base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index a1bfe11becdad..f2b29359d01a1 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -6991,7 +6991,7 @@ def infer_objects(self, copy: bool = True) -> Index:
         return result
 
     @final
-    def diff(self, periods: int = 1) -> Self:
+    def diff(self, periods: int = 1) -> Index:
         """
         Computes the difference between consecutive values in the Index object.
 

From 05b6be3dc9eaedecfdafb5b6f1fe02e2ed4a58c4 Mon Sep 17 00:00:00 2001
From: Luke Manley <lukemanley@gmail.com>
Date: Mon, 30 Oct 2023 21:01:11 -0400
Subject: [PATCH 4/4] use Index

---
 doc/source/whatsnew/v2.1.3.rst | 4 ++--
 pandas/core/indexes/base.py    | 6 +-----
 2 files changed, 3 insertions(+), 7 deletions(-)

diff --git a/doc/source/whatsnew/v2.1.3.rst b/doc/source/whatsnew/v2.1.3.rst
index ec5103c09215a..3b1cd1c152baa 100644
--- a/doc/source/whatsnew/v2.1.3.rst
+++ b/doc/source/whatsnew/v2.1.3.rst
@@ -13,7 +13,7 @@ including other versions of pandas.
 
 Fixed regressions
 ~~~~~~~~~~~~~~~~~
-- Fixed regression in :meth:`DatetimeIndex.diff` raising ``TypeError`` (:issue:`55080`)
+-
 -
 
 .. ---------------------------------------------------------------------------
@@ -21,7 +21,7 @@ Fixed regressions
 
 Bug fixes
 ~~~~~~~~~
--
+- Bug in :meth:`DatetimeIndex.diff` raising ``TypeError`` (:issue:`55080`)
 -
 
 .. ---------------------------------------------------------------------------
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index f2b29359d01a1..0301830df483b 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -7017,11 +7017,7 @@ def diff(self, periods: int = 1) -> Index:
         Index([nan, 10.0, 10.0, 10.0, 10.0], dtype='float64')
 
         """
-        diff = self.to_series().diff(periods)
-        if diff.dtype != self.dtype:
-            # e.g. DTI.diff -> TDI
-            return Index(diff)
-        return self._constructor(diff)
+        return Index(self.to_series().diff(periods))
 
     @final
     def round(self, decimals: int = 0) -> Self: