BUG: to_datetime(tz_mix, utc=True) converts to UTC (#48686)

Co-authored-by: Patrick Hoefler <61934744+phofl@users.noreply.github.com>

doc/source/whatsnew/v1.5.1.rst

@@ -72,6 +72,7 @@ Fixed regressions
 - Fixed Regression in :meth:`Series.__setitem__` casting ``None`` to ``NaN`` for object dtype (:issue:`48665`)
 - Fixed Regression in :meth:`DataFrame.loc` when setting values as a :class:`DataFrame` with all ``True`` indexer (:issue:`48701`)
 - Regression in :func:`.read_csv` causing an ``EmptyDataError`` when using an UTF-8 file handle that was already read from (:issue:`48646`)
+- Regression in :func:`to_datetime` when ``utc=True`` and ``arg`` contained timezone naive and aware arguments raised a ``ValueError`` (:issue:`48678`)
 - Fixed regression in :meth:`DataFrame.loc` raising ``FutureWarning`` when setting an empty :class:`DataFrame` (:issue:`48480`)
 - Fixed regression in :meth:`DataFrame.describe` raising ``TypeError`` when result contains ``NA`` (:issue:`48778`)
 - Fixed regression in :meth:`DataFrame.plot` ignoring invalid ``colormap`` for ``kind="scatter"`` (:issue:`48726`)

pandas/_libs/tslib.pyx

@@ -531,7 +531,7 @@ cpdef array_to_datetime(
 
                     else:
                         found_naive = True
-                        if found_tz:
+                        if found_tz and not utc_convert:
                             raise ValueError('Cannot mix tz-aware with '
                                              'tz-naive values')
                         if isinstance(val, _Timestamp):

pandas/tests/tools/test_to_datetime.py

@@ -2817,6 +2817,30 @@ def test_to_datetime_cache_coerce_50_lines_outofbounds(series_length):
         to_datetime(s, errors="raise", utc=True)
 
 
+@pytest.mark.parametrize(
+    "arg",
+    [
+        ["1724-12-20 20:20:20+00:00", "2022-01-01 00:00:00"],
+        [
+            Timestamp("1724-12-20 20:20:20+00:00"),
+            Timestamp("2022-01-01 00:00:00"),
+        ],
+        [datetime(1724, 12, 20, 20, 20, 20, tzinfo=timezone.utc), datetime(2022, 1, 1)],
+    ],
+    ids=["string", "pd.Timestamp", "datetime.datetime"],
+)
+@pytest.mark.parametrize("tz_aware_first", [True, False])
+def test_to_datetime_mixed_tzaware_timestamp_utc_true(arg, tz_aware_first):
+    # GH 48678
+    exp_arg = ["1724-12-20 20:20:20", "2022-01-01 00:00:00"]
+    if not tz_aware_first:
+        arg.reverse()
+        exp_arg.reverse()
+    result = to_datetime(arg, utc=True)
+    expected = DatetimeIndex(exp_arg).tz_localize("UTC")
+    tm.assert_index_equal(result, expected)
+
+
 def test_to_datetime_format_f_parse_nanos():
     # GH 48767
     timestamp = "15/02/2020 02:03:04.123456789"