Add wxCHECK_CXX_STD() and use it to test for C++17 and C++20

Unlike direct tests of __cplusplus, using this macro also works with
(recent enough, i.e. MSVS 2015.3 or later) MSVC versions, even if
/Zc:__cplusplus is not used.

This simplifies some checks and makes some other ones (notably the check
for C++20 used before wxALLOW_COMBINING_ENUMS macro definition) work
with MSVC versions with C++20 support as intended.

include/wx/defs.h

@@ -216,6 +216,24 @@
 #define __USE_W32_SOCKETS
 #endif
 
+#if defined(_MSVC_LANG)
+/*
+   We want to always use the really supported C++ standard when using MSVC
+   recent enough to define _MSVC_LANG, even if /Zc:__cplusplus option is not
+   used, but unfortunately we can't just redefine __cplusplus as _MSVC_LANG
+   because this is not allowed by the standard and, worse, doesn't work in
+   practice (it results in a warning and nothing else).
+
+   So, instead, we define a macro for testing __cplusplus which also works in
+   this case.
+*/
+    #define wxCHECK_CXX_STD(ver) (_MSVC_LANG >= (ver))
+#elif defined(__cplusplus)
+    #define wxCHECK_CXX_STD(ver) (__cplusplus >= (ver))
+#else
+    #define wxCHECK_CXX_STD(ver) 0
+#endif
+
 /*  ---------------------------------------------------------------------------- */
 /*  check for native bool type and TRUE/FALSE constants */
 /*  ---------------------------------------------------------------------------- */
@@ -302,7 +320,7 @@ typedef short int WXTYPE;
 
 /* wxFALLTHROUGH is used to notate explicit fallthroughs in switch statements */
 
-#if __cplusplus >= 201703L || (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
+#if wxCHECK_CXX_STD(201703L)
     #define wxFALLTHROUGH [[fallthrough]]
 #elif __cplusplus >= 201103L && defined(__has_warning) && WX_HAS_CLANG_FEATURE(cxx_attributes)
     #define wxFALLTHROUGH [[clang::fallthrough]]
@@ -1303,7 +1321,7 @@ typedef double wxDouble;
     for doing this, that do the same thing as would happen without them, but
     without the warnings.
  */
-#if defined(__cplusplus) && (__cplusplus >= 202002L)
+#if wxCHECK_CXX_STD(202002L)
     #define wxALLOW_COMBINING_ENUMS_IMPL(en1, en2)                            \
         inline int operator|(en1 v1, en2 v2)                                  \
             { return static_cast<int>(v1) | static_cast<int>(v2); }           \

include/wx/event.h

@@ -145,7 +145,7 @@ inline wxEventFunction wxEventFunctionCast(void (wxEvtHandler::*func)(T&))
 // a type of wxEvtHandler method. But with C++17 this doesn't work when the
 // handler is a noexcept function, so we need to cast it to a noexcept function
 // pointer first.
-#if __cplusplus >= 201703L
+#if wxCHECK_CXX_STD(201703L)
 
 namespace wxPrivate
 {

include/wx/math.h

@@ -59,7 +59,7 @@
     Things are simple with C++11: we have everything we need in std.
     Eventually we will only have this section and not the legacy stuff below.
  */
-#if __cplusplus >= 201103
+#if wxCHECK_CXX_STD(201103)
     #define wxFinite(x) std::isfinite(x)
     #define wxIsNaN(x) std::isnan(x)
 #else /* C++98 */
@@ -139,7 +139,7 @@ inline int wxRound(double x)
     wxASSERT_MSG(x > double(INT_MIN) - 0.5 && x < double(INT_MAX) + 0.5,
         "argument out of supported range");
 
-    #if __cplusplus >= 201103
+    #if wxCHECK_CXX_STD(201103)
         return int(std::lround(x));
     #elif defined(HAVE_ROUND) || wxCHECK_VISUALC_VERSION(12)
         return int(lround(x));
@@ -153,7 +153,7 @@ inline int wxRound(float x)
     wxASSERT_MSG(x > float(INT_MIN) && x < float(INT_MAX),
         "argument out of supported range");
 
-    #if __cplusplus >= 201103
+    #if wxCHECK_CXX_STD(201103)
         return int(std::lround(x));
     #elif defined(HAVE_ROUND) || wxCHECK_VISUALC_VERSION(12)
         return int(lroundf(x));

include/wx/wxcrtbase.h

@@ -180,7 +180,7 @@ extern unsigned long android_wcstoul(const wchar_t *nptr, wchar_t **endptr, int
     #define wxCRT_StrtoullW  _wcstoui64
 #else
     /* Both of these functions are implemented in C++11 compilers */
-    #if defined(__cplusplus) && __cplusplus >= 201103L
+    #if wxCHECK_CXX_STD(201103L)
         #ifndef HAVE_STRTOULL
             #define HAVE_STRTOULL
         #endif

interface/wx/defs.h

@@ -1516,6 +1516,26 @@ typedef double wxDouble;
 /** @addtogroup group_funcmacro_misc */
 //@{
 
+/**
+    Returns @true if the compiler being used supports the given C++ version.
+
+    The @a stdver parameter uses the same values as the standard @c __cplusplus
+    macro, i.e.
+
+    - 201103L for C++11
+    - 201402L for C++14
+    - 201703L for C++17
+    - 202002L for C++20
+
+    Using this macro also works with MSVC, even when @c /Zc:__cplusplus option
+    is not used, unlike checking for the value of @c __cplusplus directly.
+
+    @since 3.1.7
+
+    @header{wx/defs.h}
+*/
+#define wxCHECK_CXX_STD(stdver)
+
 /**
     This macro can be used in a class declaration to disable the generation of
     default assignment operator.

tests/test.cpp

@@ -133,7 +133,7 @@ static void TestAssertHandler(const wxString& file,
         // so we'd just die without any useful information -- abort instead.
         abortReason << assertMessage << wxASCII_STR(" in a worker thread.");
     }
-#if __cplusplus >= 201703L || wxCHECK_VISUALC_VERSION(14)
+#if wxCHECK_CXX_STD(201703L)
     else if ( uncaught_exceptions() )
 #else
     else if ( uncaught_exception() )