ENH: handle nonexistent shift for ZoneInfo

pandas/_libs/tslibs/tzconversion.pyx

@@ -240,6 +240,8 @@ timedelta-like}
         str stamp
         Localizer info = Localizer(tz, creso=creso)
         int64_t pph = periods_per_day(creso) // 24
+        int64_t pps = periods_per_second(creso)
+        npy_datetimestruct dts
 
     # Vectorized version of DstTzInfo.localize
     if info.use_utc:
@@ -388,14 +390,26 @@ timedelta-like}
                     new_local = val - remaining_mins - 1
 
                 if is_zi:
-                    raise NotImplementedError(
-                        "nonexistent shifting is not implemented with ZoneInfo tzinfos"
-                    )
+                    # use the same construction as in _get_utc_bounds_zoneinfo
+                    pandas_datetime_to_datetimestruct(new_local, creso, &dts)
+                    extra = (dts.ps // 1000) * (pps // 1_000_000_000)
+
+                    dt = datetime_new(dts.year, dts.month, dts.day, dts.hour,
+                                      dts.min, dts.sec, dts.us, None)
 
-                delta_idx = bisect_right_i8(info.tdata, new_local, info.ntrans)
+                    if shift_forward or shift_delta > 0:
+                        dt = dt.replace(tzinfo=tz, fold=1)
+                    else:
+                        dt = dt.replace(tzinfo=tz, fold=0)
+                    dt = dt.astimezone(utc_stdlib)
+                    dt = dt.replace(tzinfo=None)
+                    result[i] = pydatetime_to_dt64(dt, &dts, creso) + extra
+
+                else:
+                    delta_idx = bisect_right_i8(info.tdata, new_local, info.ntrans)
 
-                delta_idx = delta_idx - delta_idx_offset
-                result[i] = new_local - info.deltas[delta_idx]
+                    delta_idx = delta_idx - delta_idx_offset
+                    result[i] = new_local - info.deltas[delta_idx]
             elif fill_nonexist:
                 result[i] = NPY_NAT
             else:

pandas/tests/indexes/datetimes/test_constructors.py

@@ -880,15 +880,8 @@ def test_constructor_with_ambiguous_keyword_arg(self):
         result = date_range(end=end, periods=2, ambiguous=False)
         tm.assert_index_equal(result, expected)
 
-    def test_constructor_with_nonexistent_keyword_arg(self, warsaw, request):
+    def test_constructor_with_nonexistent_keyword_arg(self, warsaw):
         # GH 35297
-        if type(warsaw).__name__ == "ZoneInfo":
-            mark = pytest.mark.xfail(
-                reason="nonexistent-shift not yet implemented for ZoneInfo",
-                raises=NotImplementedError,
-            )
-            request.node.add_marker(mark)
-
         timezone = warsaw
 
         # nonexistent keyword in start

pandas/tests/series/methods/test_tz_localize.py

@@ -64,6 +64,7 @@ def test_series_tz_localize_matching_index(self):
         "method, exp",
         [
             ["shift_forward", "2015-03-29 03:00:00"],
+            ["shift_backward", "2015-03-29 01:59:59.999999999"],
             ["NaT", NaT],
             ["raise", None],
             ["foo", "invalid"],
@@ -99,15 +100,6 @@ def test_tz_localize_nonexistent(self, warsaw, method, exp):
             with pytest.raises(ValueError, match=msg):
                 df.tz_localize(tz, nonexistent=method)
 
-        elif method == "shift_forward" and type(tz).__name__ == "ZoneInfo":
-            msg = "nonexistent shifting is not implemented with ZoneInfo tzinfos"
-            with pytest.raises(NotImplementedError, match=msg):
-                ser.tz_localize(tz, nonexistent=method)
-            with pytest.raises(NotImplementedError, match=msg):
-                df.tz_localize(tz, nonexistent=method)
-            with pytest.raises(NotImplementedError, match=msg):
-                dti.tz_localize(tz, nonexistent=method)
-
         else:
             result = ser.tz_localize(tz, nonexistent=method)
             expected = Series(1, index=DatetimeIndex([exp] * n, tz=tz))