DEPR: tz_convert in the Timestamp constructor

doc/source/whatsnew/v0.24.0.txt

@@ -967,6 +967,7 @@ Deprecations
 - The class ``FrozenNDArray`` has been deprecated. When unpickling, ``FrozenNDArray`` will be unpickled to ``np.ndarray`` once this class is removed (:issue:`9031`)
 - Deprecated the `nthreads` keyword of :func:`pandas.read_feather` in favor of
   `use_threads` to reflect the changes in pyarrow 0.11.0. (:issue:`23053`)
+- Timezone converting a tz-aware ``datetime.datetime`` or :class:`Timestamp` with :class:`Timestamp` and the ``tz`` argument is now deprecated. Instead use :meth:`Timestamp.tz_convert` (:issue:`23579`)
 
 .. _whatsnew_0240.deprecations.datetimelike_int_ops:
 

pandas/_libs/tslibs/timestamps.pyx

@@ -709,9 +709,6 @@ class Timestamp(_Timestamp):
 
         elif ts_input is _no_input:
             # User passed keyword arguments.
-            if tz is None:
-                # Handle the case where the user passes `tz` and not `tzinfo`
-                tz = tzinfo
             return Timestamp(datetime(year, month, day, hour or 0,
                                       minute or 0, second or 0,
                                       microsecond or 0, tzinfo),
@@ -728,6 +725,11 @@ class Timestamp(_Timestamp):
             # User passed tzinfo instead of tz; avoid silently ignoring
             tz, tzinfo = tzinfo, None
 
+        if getattr(ts_input, 'tzinfo', None) is not None and tz is not None:
+            warnings.warn("Passing a datetime or Timestamp with tzinfo and the"
+                          " tz parameter will raise in the future. Use"
+                          " tz_convert instead.", FutureWarning)
+
         ts = convert_to_tsobject(ts_input, tz, unit, 0, 0, nanosecond or 0)
 
         if ts.value == NPY_NAT:

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -244,7 +244,10 @@ def test_constructor(self):
                     assert conversion.pydt_to_i8(result) == expected_tz
 
                     # should convert to UTC
-                    result = Timestamp(result, tz='UTC')
+                    if tz is not None:
+                        result = Timestamp(result).tz_convert('UTC')
+                    else:
+                        result = Timestamp(result, tz='UTC')
                     expected_utc = expected - offset * 3600 * 1000000000
                     assert result.value == expected_utc
                     assert conversion.pydt_to_i8(result) == expected_utc
@@ -295,7 +298,7 @@ def test_constructor_with_stringoffset(self):
                 assert conversion.pydt_to_i8(result) == expected_tz
 
                 # should convert to UTC
-                result = Timestamp(result, tz='UTC')
+                result = Timestamp(result).tz_convert('UTC')
                 expected_utc = expected
                 assert result.value == expected_utc
                 assert conversion.pydt_to_i8(result) == expected_utc
@@ -558,7 +561,7 @@ def test_construct_timestamp_near_dst(self, offset):
         # GH 20854
         expected = Timestamp('2016-10-30 03:00:00{}'.format(offset),
                              tz='Europe/Helsinki')
-        result = Timestamp(expected, tz='Europe/Helsinki')
+        result = Timestamp(expected).tz_convert('Europe/Helsinki')
         assert result == expected
 
     @pytest.mark.parametrize('arg', [
@@ -580,6 +583,13 @@ def test_constructor_invalid_frequency(self):
         with tm.assert_raises_regex(ValueError, "Invalid frequency:"):
             Timestamp('2012-01-01', freq=[])
 
+    @pytest.mark.parametrize('box', [datetime, Timestamp])
+    def test_depreciate_tz_and_tzinfo_in_datetime_input(self, box):
+        # GH 23579
+        kwargs = {'year': 2018, 'month': 1, 'day': 1, 'tzinfo': utc}
+        with tm.assert_produces_warning(FutureWarning):
+            Timestamp(box(**kwargs), tz='US/Pacific')
+
 
 class TestTimestamp(object):