Build system: support C++2c

CMake supports it since 3.25 (and we ship Qt with 3.30), so allow the
selection of to-be-C++26.

This patch merely continues the status quo. It does not attempt to
change policy.

Pick-to: 6.10 6.9 6.8 6.5
Change-Id: I6e1e7d76f787d619ad16551114ca92423159917a
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

cmake/QtFeature.cmake

@@ -1570,7 +1570,8 @@ function(qt_run_config_compile_test name)
             endif()
 
             if(arg_CXX_STANDARD)
-                if(${arg_CXX_STANDARD} LESS 23 OR ${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.20")
+                if((${arg_CXX_STANDARD} LESS 23 OR ${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.20") AND
+                   (${arg_CXX_STANDARD} LESS 26 OR ${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.25"))
                     set(CMAKE_CXX_STANDARD "${arg_CXX_STANDARD}")
                     set(CMAKE_CXX_STANDARD_REQUIRED OFF)
                 endif()

cmake/QtFlagHandlingHelpers.cmake

@@ -359,7 +359,9 @@ endfunction()
 
 function(qt_set_language_standards)
     ## Use the latest standard the compiler supports (same as qt_common.prf)
-    if (QT_FEATURE_cxx2b)
+    if (QT_FEATURE_cxx2c)
+        set(CMAKE_CXX_STANDARD 26 PARENT_SCOPE)
+    elseif (QT_FEATURE_cxx2b)
         set(CMAKE_CXX_STANDARD 23 PARENT_SCOPE)
     elseif (QT_FEATURE_cxx20)
         set(CMAKE_CXX_STANDARD 20 PARENT_SCOPE)

configure.cmake

@@ -292,6 +292,25 @@ int main(void)
     CXX_STANDARD 23
 )
 
+qt_config_compile_test(cxx2c
+    LABEL "C++2c support"
+    CODE
+"#if __cplusplus > 202302L
+// Compiler claims to support C++2c, trust it
+#else
+#  error __cplusplus must be > 202302L (the value for C++23)
+#endif
+
+int main(void)
+{
+    /* BEGIN TEST: */
+    /* END TEST: */
+    return 0;
+}
+"
+    CXX_STANDARD 26
+)
+
 qt_config_compiler_supports_flag_test(optimize_debug
     LABEL "-Og support"
     FLAG "-Og"
@@ -730,6 +749,12 @@ qt_feature("c++2b" PUBLIC
     CONDITION QT_FEATURE_cxx20 AND (CMAKE_VERSION VERSION_GREATER_EQUAL "3.20") AND TEST_cxx2b
 )
 qt_feature_config("c++2b" QMAKE_PUBLIC_QT_CONFIG)
+qt_feature("c++2c" PUBLIC
+    LABEL "C++2c"
+    AUTODETECT OFF
+    CONDITION QT_FEATURE_cxx2b AND (CMAKE_VERSION VERSION_GREATER_EQUAL "3.25") AND TEST_cxx2c
+)
+qt_feature_config("c++2c" QMAKE_PUBLIC_QT_CONFIG)
 set(__qt_ltcg_detected FALSE)
 if(CMAKE_INTERPROCEDURAL_OPTIMIZATION)
     set(__qt_ltcg_detected TRUE)

mkspecs/common/clang.conf

@@ -36,11 +36,13 @@ QMAKE_CXXFLAGS_CXX14             = -std=c++1y
 QMAKE_CXXFLAGS_CXX1Z             = -std=c++1z
 QMAKE_CXXFLAGS_CXX2A             = -std=c++2a
 QMAKE_CXXFLAGS_CXX2B             = -std=c++2b
+QMAKE_CXXFLAGS_CXX2C             = -std=c++2c
 QMAKE_CXXFLAGS_GNUCXX11          = -std=gnu++11
 QMAKE_CXXFLAGS_GNUCXX14          = -std=gnu++1y
 QMAKE_CXXFLAGS_GNUCXX1Z          = -std=gnu++1z
 QMAKE_CXXFLAGS_GNUCXX2A          = -std=gnu++2a
 QMAKE_CXXFLAGS_GNUCXX2B          = -std=gnu++2b
+QMAKE_CXXFLAGS_GNUCXX2C          = -std=gnu++2c
 
 QMAKE_LFLAGS_CXX11      =
 QMAKE_LFLAGS_CXX14      =

mkspecs/common/g++-base.conf

@@ -34,11 +34,13 @@ QMAKE_CXXFLAGS_CXX14    = -std=c++1y
 QMAKE_CXXFLAGS_CXX1Z    = -std=c++1z
 QMAKE_CXXFLAGS_CXX2A    = -std=c++2a
 QMAKE_CXXFLAGS_CXX2B    = -std=c++2b
+QMAKE_CXXFLAGS_CXX2C    = -std=c++2c
 QMAKE_CXXFLAGS_GNUCXX11 = -std=gnu++11
 QMAKE_CXXFLAGS_GNUCXX14 = -std=gnu++1y
 QMAKE_CXXFLAGS_GNUCXX1Z = -std=gnu++1z
 QMAKE_CXXFLAGS_GNUCXX2A = -std=gnu++2a
 QMAKE_CXXFLAGS_GNUCXX2B = -std=gnu++2b
+QMAKE_CXXFLAGS_GNUCXX2C = -std=gnu++2c
 QMAKE_LFLAGS_CXX11      =
 QMAKE_LFLAGS_CXX14      =
 QMAKE_LFLAGS_CXX1Z      =

mkspecs/common/msvc-version.conf

@@ -121,6 +121,7 @@ greaterThan(QMAKE_MSC_VER, 1919) {
     MSVC_TOOLSET_VER    = 142
     QMAKE_CXXFLAGS_CXX2A = -std:c++latest
     QMAKE_CXXFLAGS_CXX2B = -std:c++latest
+    QMAKE_CXXFLAGS_CXX2C = -std:c++latest
 
     QMAKE_CXXFLAGS     += -Zc:externConstexpr
 }

mkspecs/features/default_post.prf

@@ -124,17 +124,18 @@ breakpad {
 c++17: CONFIG += c++1z
 c++20: CONFIG += c++2a
 c++23: CONFIG += c++2b
-c++latest: CONFIG *= c++2b c++2a c++1z c++14 c++11
+c++latest: CONFIG *= c++2c c++2b c++2a c++1z c++14 c++11
 
-!c++1z:!c++2a:!c++2b {
+!c++1z:!c++2a:!c++2b:!c++2c {
     # Qt requires C++17
     QT_COMPILER_STDCXX_no_L = $$replace(QT_COMPILER_STDCXX, "L$", "")
     !greaterThan(QT_COMPILER_STDCXX_no_L, 201402): CONFIG += c++1z
 }
-c++1z|c++2a|c++2b {
+c++1z|c++2a|c++2b|c++2c {
     # Disable special compiler flags for host builds
     !host_build|!cross_compile {
-        c++2b:       cxxstd = CXX2B
+        c++2c:       cxxstd = CXX2C
+        else:c++2b:  cxxstd = CXX2B
         else:c++2a:  cxxstd = CXX2A
         else:        cxxstd = CXX1Z
     } else {

qt_cmdline.cmake

@@ -174,12 +174,19 @@ function(qt_commandline_cxxstd arg val nextok)
     if(val MATCHES "(c\\+\\+)?(17|1z)")
         qtConfCommandlineDisableFeature(c++20)
         qtConfCommandlineDisableFeature(c++2b)
+        qtConfCommandlineDisableFeature(c++2c)
     elseif(val MATCHES "(c\\+\\+)?(20|2a)")
         qtConfCommandlineEnableFeature(c++20)
         qtConfCommandlineDisableFeature(c++2b)
+        qtConfCommandlineDisableFeature(c++2c)
     elseif(val MATCHES "(c\\+\\+)?(23|2b)")
         qtConfCommandlineEnableFeature(c++20)
         qtConfCommandlineEnableFeature(c++2b)
+        qtConfCommandlineDisableFeature(c++2c)
+    elseif(val MATCHES "(c\\+\\+)?(2c)")
+        qtConfCommandlineEnableFeature(c++20)
+        qtConfCommandlineEnableFeature(c++2b)
+        qtConfCommandlineEnableFeature(c++2c)
     else()
         qtConfAddError("Invalid argument '${val}' to command line parameter '${arg}'")
     endif()