Operating system: Win10 Pro v1903 OS build: 18362.778
wxPython version & source: 4.1.0 msw (phoenix)
Python version & source: 3.6.8 64 bit from python.org

Description of the problem:

The 4.1.0 includes an (unexpected?) breaking change that now throws errors for certain alignment flags. This causes applications to break when updating from 4.0.7 to 4.1.0.

I'm curious if this was a planned breakage? I've looked over the release notes, but I don't see anything specifically called out as such. Similarly, running with PYTHONWARNINGS=default on 4.0.x shows no relevant warnings in my code base for this specific issue.

The issue can be reproduced with the following code:

import wx

app = wx.App()

frame = wx.Frame(None, title='Simple application')
sizer = wx.BoxSizer(wx.HORIZONTAL)
sizer.Add(wx.StaticText(frame, -1, "foo"), 1, wx.ALIGN_CENTER_HORIZONTAL)

frame.Show()
app.MainLoop()

Traceback:

Traceback (most recent call last):
  File "Gooey/gooey/examples/wxissue.py", line 7, in <module>
    sizer.Add(wx.StaticText(frame, -1, "foo"), 1, wx.ALIGN_CENTER_HORIZONTAL)
wx._core.wxAssertionError: C++ assertion "!(flags & wxALIGN_CENTRE_HORIZONTAL)" failed at ..\..\src\common\sizer.cpp(2106) in wxBoxSizer::DoInsert(): Horizontal alignment flags are ignored in horizontal sizers