C++23/c++2b support

Change-Id: I33b2a48312ae94e3d5ebb4097e50c4953e14d533
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

cmake/QtFeature.cmake

@@ -938,8 +938,10 @@ function(qt_config_compile_test name)
             endif()
 
             if(arg_CXX_STANDARD)
+                if(${arg_CXX_STANDARD} LESS 23 OR ${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.20")
                     set(CMAKE_CXX_STANDARD "${arg_CXX_STANDARD}")
                 endif()
+            endif()
 
             set(CMAKE_REQUIRED_FLAGS ${arg_COMPILE_OPTIONS})
 

cmake/QtFlagHandlingHelpers.cmake

@@ -230,7 +230,9 @@ endfunction()
 
 function(qt_set_language_standards)
     ## Use the latest standard the compiler supports (same as qt_common.prf)
-    if (QT_FEATURE_cxx20)
+    if (QT_FEATURE_cxx2b)
+        set(CMAKE_CXX_STANDARD 23 PARENT_SCOPE)
+    elseif (QT_FEATURE_cxx20)
         set(CMAKE_CXX_STANDARD 20 PARENT_SCOPE)
     else()
         set(CMAKE_CXX_STANDARD 17 PARENT_SCOPE)

config_help.txt

@@ -110,7 +110,7 @@ Build options:
                          for example, -sanitize address cannot be combined with
                          -sanitize thread.
 
-  -c++std <edition> .... Select C++ standard <edition> [c++20/c++17/c++14/c++11]
+  -c++std <edition> .... Select C++ standard <edition> [c++2b/c++20/c++17/c++14/c++11]
 
   -sse2 ................ Use SSE2 instructions [auto]
   -sse3/-ssse3/-sse4.1/-sse4.2/-avx/-avx2/-avx512

configure.cmake

@@ -177,6 +177,25 @@ int main(void)
     CXX_STANDARD 20
 )
 
+qt_config_compile_test(cxx2b
+    LABEL "C++2b support"
+    CODE
+"#if __cplusplus > 202002L
+// Compiler claims to support C++2B, trust it
+#else
+#  error __cplusplus must be > 202002L (the value for C++20)
+#endif
+
+int main(void)
+{
+    /* BEGIN TEST: */
+    /* END TEST: */
+    return 0;
+}
+"
+    CXX_STANDARD 23
+)
+
 # precompile_header
 qt_config_compile_test(precompile_header
     LABEL "precompiled header support"
@@ -642,6 +661,11 @@ qt_feature("c++2b" PUBLIC
     AUTODETECT OFF
 )
 qt_feature_config("c++2b" QMAKE_PUBLIC_QT_CONFIG)
+qt_feature("c++2b" PUBLIC
+    LABEL "C++2b"
+    AUTODETECT FALSE
+    CONDITION QT_FEATURE_cxx20 AND (CMAKE_VERSION VERSION_GREATER_EQUAL "3.20") AND TEST_cxx2b
+)
 qt_feature("c89"
     LABEL "C89"
 )
@@ -1061,7 +1085,7 @@ qt_configure_add_summary_entry(
 )
 qt_configure_add_summary_entry(
     TYPE "firstAvailableFeature"
-    ARGS "c++20 c++17 c++14 c++11"
+    ARGS "c++2b c++20 c++17 c++14 c++11"
     MESSAGE "Using C++ standard"
 )
 qt_configure_add_summary_entry(

qt_cmdline.cmake

@@ -138,18 +138,27 @@ function(qt_commandline_cxxstd arg val nextok)
         qtConfCommandlineSetInput(c++14 no)
         qtConfCommandlineSetInput(c++17 no)
         qtConfCommandlineSetInput(c++20 no)
+        qtConfCommandlineSetInput(c++2b no)
     elseif(val MATCHES "(c\\+\\+)?(14|1y)")
         qtConfCommandlineSetInput(c++14 yes)
         qtConfCommandlineSetInput(c++17 no)
         qtConfCommandlineSetInput(c++20 no)
+        qtConfCommandlineSetInput(c++2b no)
     elseif(val MATCHES "(c\\+\\+)?(17|1z)")
         qtConfCommandlineSetInput(c++14 yes)
         qtConfCommandlineSetInput(c++17 yes)
         qtConfCommandlineSetInput(c++20 no)
+        qtConfCommandlineSetInput(c++2b no)
     elseif(val MATCHES "(c\\+\\+)?(20|2a)")
         qtConfCommandlineSetInput(c++14 yes)
         qtConfCommandlineSetInput(c++17 yes)
         qtConfCommandlineSetInput(c++20 yes)
+        qtConfCommandlineSetInput(c++2b no)
+    elseif(val MATCHES "(c\\+\\+)?(2b)")
+        qtConfCommandlineSetInput(c++14 yes)
+        qtConfCommandlineSetInput(c++17 yes)
+        qtConfCommandlineSetInput(c++20 yes)
+        qtConfCommandlineSetInput(c++2b yes)
     else()
         qtConfAddError("Invalid argument '${val}' to command line parameter '${arg}'")
     endif()