diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7a5d8b9b0b..ab8bf3804f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -88,6 +88,10 @@ jobs:
             skip_samples: true
             use_asan: true
             use_xvfb: true
+          - name: Ubuntu 22.04 wxGTK C++20
+            runner: ubuntu-22.04
+            configure_flags: --with-cxx=20
+            skip_samples: true
           - name: Ubuntu 18.04 wxX11
             runner: ubuntu-18.04
             configure_flags: --with-x11 --enable-pch --disable-stc
diff --git a/include/wx/defs.h b/include/wx/defs.h
index cc4bcf07be..400404dd7a 100644
--- a/include/wx/defs.h
+++ b/include/wx/defs.h
@@ -1465,12 +1465,21 @@ enum wxBorder
     Elements of these enums can be combined with each other when using
     wxSizer::Add() overload not using wxSizerFlags.
  */
+wxALLOW_COMBINING_ENUMS(wxAlignment, wxBorder)
 wxALLOW_COMBINING_ENUMS(wxAlignment, wxDirection)
 wxALLOW_COMBINING_ENUMS(wxAlignment, wxGeometryCentre)
+wxALLOW_COMBINING_ENUMS(wxAlignment, wxSizerFlagBits)
 wxALLOW_COMBINING_ENUMS(wxAlignment, wxStretch)
-wxALLOW_COMBINING_ENUMS(wxAlignment, wxBorder)
-wxALLOW_COMBINING_ENUMS(wxDirection, wxStretch)
+wxALLOW_COMBINING_ENUMS(wxBorder, wxDirection)
+wxALLOW_COMBINING_ENUMS(wxBorder, wxGeometryCentre)
+wxALLOW_COMBINING_ENUMS(wxBorder, wxSizerFlagBits)
+wxALLOW_COMBINING_ENUMS(wxBorder, wxStretch)
 wxALLOW_COMBINING_ENUMS(wxDirection, wxGeometryCentre)
+wxALLOW_COMBINING_ENUMS(wxDirection, wxStretch)
+wxALLOW_COMBINING_ENUMS(wxDirection, wxSizerFlagBits)
+wxALLOW_COMBINING_ENUMS(wxGeometryCentre, wxSizerFlagBits)
+wxALLOW_COMBINING_ENUMS(wxGeometryCentre, wxStretch)
+wxALLOW_COMBINING_ENUMS(wxSizerFlagBits, wxStretch)
 
 /*  ---------------------------------------------------------------------------- */
 /*  Window style flags */
diff --git a/tests/regex/wxregextest.cpp b/tests/regex/wxregextest.cpp
index e1d58d1f65..291ed7afb1 100644
--- a/tests/regex/wxregextest.cpp
+++ b/tests/regex/wxregextest.cpp
@@ -71,12 +71,12 @@ static void
 CheckMatch(const char* pattern,
            const char* text,
            const char* expected = NULL,
-           int flags = wxRE_DEFAULT)
+           int compileFlags = wxRE_DEFAULT,
+           int matchFlags = 0)
 {
-    int compileFlags = flags & ~(wxRE_NOTBOL | wxRE_NOTEOL);
-    int matchFlags = flags & (wxRE_NOTBOL | wxRE_NOTEOL);
-
-    INFO( "Pattern: " << pattern << FlagStr(flags) << ", match: " << text );
+    INFO( "Pattern: "
+            << pattern << FlagStr(static_cast<int>(compileFlags) | matchFlags)
+            << ", match: " << text );
 
     wxRegEx re(pattern, compileFlags);
     if ( !re.IsValid() )
@@ -104,7 +104,7 @@ CheckMatch(const char* pattern,
         CHECK( re.GetMatch(text, i) == tkz.GetNextToken() );
     }
 
-    if ((flags & wxRE_NOSUB) == 0)
+    if ((compileFlags & wxRE_NOSUB) == 0)
         CHECK(re.GetMatchCount() == i);
 }
 
@@ -124,8 +124,8 @@ TEST_CASE("wxRegEx::Match", "[regex][match]")
     CheckMatch("^[A-Z].*$", "AA\nbb\nCC", "AA\nbb\nCC");
     CheckMatch("^[A-Z].*$", "AA\nbb\nCC", "AA", wxRE_NEWLINE);
     CheckMatch("^[a-z].*$", "AA\nbb\nCC", "bb", wxRE_NEWLINE);
-    CheckMatch("^[A-Z].*$", "AA\nbb\nCC", "CC", wxRE_NEWLINE | wxRE_NOTBOL);
-    CheckMatch("^[A-Z].*$", "AA\nbb\nCC", NULL, wxRE_NEWLINE | wxRE_NOTBOL | wxRE_NOTEOL);
+    CheckMatch("^[A-Z].*$", "AA\nbb\nCC", "CC", wxRE_NEWLINE, wxRE_NOTBOL);
+    CheckMatch("^[A-Z].*$", "AA\nbb\nCC", NULL, wxRE_NEWLINE, wxRE_NOTBOL | wxRE_NOTEOL);
     CheckMatch("([[:alpha:]]+) ([[:alpha:]]+) ([[:digit:]]+).* ([[:digit:]]+)$",
         "Fri Jul 13 18:37:52 CEST 2001",
         "Fri Jul 13 18:37:52 CEST 2001\tFri\tJul\t13\t2001");
diff --git a/tests/sizers/boxsizer.cpp b/tests/sizers/boxsizer.cpp
index 49e71a3c69..c566364284 100644
--- a/tests/sizers/boxsizer.cpp
+++ b/tests/sizers/boxsizer.cpp
@@ -447,3 +447,39 @@ TEST_CASE_METHOD(BoxSizerTestCase, "BoxSizer::Replace", "[sizer]")
     m_sizer->AddSpacer(1);
     m_sizer->Replace(0, new wxSizerItem(new wxWindow(m_win, wxID_ANY)));
 }
+
+TEST_CASE("Sizer::CombineFlags", "[sizer]")
+{
+    // This is a compile-time test which simply verifies that we can combine
+    // all the different flags without getting any warnings about doing it --
+    // as would have been the case when using C++20 or later if we didn't use
+    // wxALLOW_COMBINING_ENUMS() for all these enums in wx/defs.h.
+    //
+    // These constants belong to the following enums, respectively:
+    //
+    //         wxALIGN_CENTER         wxAlignment
+    //         wxBORDER_NONE          wxBorder
+    //         wxLEFT                 wxDirection
+    //         wxCENTER               wxGeometryCentre
+    //         wxFIXED_MINSIZE        wxSizerFlagBits
+    //         wxEXPAND               wxStretch
+    //
+    int n = (wxALIGN_CENTER | wxBORDER_NONE)
+          | (wxALIGN_CENTER | wxLEFT)
+          | (wxALIGN_CENTER | wxCENTER)
+          | (wxALIGN_CENTER | wxFIXED_MINSIZE)
+          | (wxALIGN_CENTER | wxEXPAND)
+          | (wxBORDER_NONE | wxLEFT)
+          | (wxBORDER_NONE | wxCENTER)
+          | (wxBORDER_NONE | wxFIXED_MINSIZE)
+          | (wxBORDER_NONE | wxEXPAND)
+          | (wxLEFT | wxCENTER)
+          | (wxLEFT | wxFIXED_MINSIZE)
+          | (wxLEFT | wxEXPAND)
+          | (wxCENTER | wxFIXED_MINSIZE)
+          | (wxCENTER | wxEXPAND)
+          | (wxFIXED_MINSIZE | wxEXPAND)
+          ;
+
+    wxUnusedVar(n);
+}