diff --git a/include/wx/aui/auibook.h b/include/wx/aui/auibook.h
index dc3f509510..23d66ade71 100644
--- a/include/wx/aui/auibook.h
+++ b/include/wx/aui/auibook.h
@@ -55,6 +55,7 @@ enum wxAuiNotebookOption
                              wxAUI_NB_MIDDLE_CLICK_CLOSE
 };
 
+wxALLOW_COMBINING_ENUMS(wxAuiNotebookOption, wxBorder)
 
 
 
diff --git a/include/wx/defs.h b/include/wx/defs.h
index d98dac5354..07533d00f6 100644
--- a/include/wx/defs.h
+++ b/include/wx/defs.h
@@ -1255,6 +1255,28 @@ typedef double wxDouble;
 /*  Geometric flags */
 /*  ---------------------------------------------------------------------------- */
 
+/*
+    In C++20 operations on the elements of different enums are deprecated and
+    many compilers (clang 10+, gcc 11+, MSVS 2019) warn about combining them,
+    as a lot of existing code using them does, so we provide explicit operators
+    for doing this, that do the same thing as would happen without them, but
+    without the warnings.
+ */
+#if defined(__cplusplus) && (__cplusplus >= 202002L)
+    #define wxALLOW_COMBINING_ENUMS_IMPL(en1, en2)                            \
+        inline int operator|(en1 v1, en2 v2)                                  \
+            { return static_cast<int>(v1) | static_cast<int>(v2); }           \
+        inline int operator+(en1 v1, en2 v2)                                  \
+            { return static_cast<int>(v1) + static_cast<int>(v2); }
+
+    #define wxALLOW_COMBINING_ENUMS(en1, en2)                                 \
+        wxALLOW_COMBINING_ENUMS_IMPL(en1, en2)                                \
+        wxALLOW_COMBINING_ENUMS_IMPL(en2, en1)
+#else /* !C++ 20 */
+    /* Don't bother doing anything in this case. */
+    #define wxALLOW_COMBINING_ENUMS(en1, en2)
+#endif /* C++ 20 */
+
 enum wxGeometryCentre
 {
     wxCENTRE                  = 0x0001,
@@ -1380,6 +1402,16 @@ enum wxBorder
 /* This makes it easier to specify a 'normal' border for a control */
 #define wxDEFAULT_CONTROL_BORDER    wxBORDER_SUNKEN
 
+/*
+    Elements of these enums can be combined with each other when using
+    wxSizer::Add() overload not using wxSizerFlags.
+ */
+wxALLOW_COMBINING_ENUMS(wxAlignment, wxDirection)
+wxALLOW_COMBINING_ENUMS(wxAlignment, wxGeometryCentre)
+wxALLOW_COMBINING_ENUMS(wxAlignment, wxStretch)
+wxALLOW_COMBINING_ENUMS(wxDirection, wxStretch)
+wxALLOW_COMBINING_ENUMS(wxDirection, wxGeometryCentre)
+
 /*  ---------------------------------------------------------------------------- */
 /*  Window style flags */
 /*  ---------------------------------------------------------------------------- */
diff --git a/include/wx/richtext/richtextbuffer.h b/include/wx/richtext/richtextbuffer.h
index 13e1dfb867..132667c50c 100644
--- a/include/wx/richtext/richtextbuffer.h
+++ b/include/wx/richtext/richtextbuffer.h
@@ -345,6 +345,8 @@ enum wxTextBoxAttrPosition
     wxTEXT_BOX_ATTR_POSITION_MASK           = 0x00F0
 };
 
+wxALLOW_COMBINING_ENUMS(wxTextAttrUnits, wxTextAttrValueFlags)
+
 /**
     @class wxTextAttrDimension
 
diff --git a/include/wx/toolbar.h b/include/wx/toolbar.h
index 3aacb1f27d..f6a3a766f0 100644
--- a/include/wx/toolbar.h
+++ b/include/wx/toolbar.h
@@ -17,7 +17,7 @@
 // wxToolBar style flags
 // ----------------------------------------------------------------------------
 
-enum
+enum wxToolBarStyleFlags
 {
     // lay out the toolbar horizontally
     wxTB_HORIZONTAL  = wxHORIZONTAL,    // == 0x0004
@@ -61,6 +61,8 @@ enum
     wxTB_DEFAULT_STYLE = wxTB_HORIZONTAL
 };
 
+wxALLOW_COMBINING_ENUMS(wxToolBarStyleFlags, wxBorder)
+
 #if wxUSE_TOOLBAR
     #include "wx/tbarbase.h"     // the base class for all toolbars
 
diff --git a/src/common/tarstrm.cpp b/src/common/tarstrm.cpp
index 30b584e41d..7324b3ac3c 100644
--- a/src/common/tarstrm.cpp
+++ b/src/common/tarstrm.cpp
@@ -58,9 +58,7 @@ enum {
     TAR_NUMFIELDS
 };
 
-enum {
-    TAR_BLOCKSIZE = 512
-};
+static const int TAR_BLOCKSIZE = 512;
 
 // checksum type
 enum {