qmake: Add support for C++23

Add the CONFIG value c++2b that represent the upcoming C++23 standard.
Add QMAKE_CXXFLAGS_CXX2B and QMAKE_CXXFLAGS_GNUCXX2B.
On MSVC, use /std:c++latest when c++2b is active.

Task-number: QTBUG-102202
Change-Id: Ie00ee5793c1a649195013c8c19efc8d59cf0acc9
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
(cherry picked from commit b5ed3cb7baeb6d77c375134719ee04ffcc211b9f)
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

config_help.txt

@@ -142,7 +142,7 @@ Build options:
                          sanitizers or generate instrumented code to collect execution
                          counts and enable code coverage analysis, respectively. (Clang only)
 
-  -c++std <edition> .... Select C++ standard <edition> [c++2a/c++17/c++14/c++11]
+  -c++std <edition> .... Select C++ standard <edition> [c++2b/c++2a/c++17/c++14/c++11]
                          (Not supported with MSVC 2015)
 
   -sse2 ................ Use SSE2 instructions [auto]

configure.json

@@ -380,6 +380,20 @@
                 "qmake": "CONFIG += c++11 c++14 c++17 c++2a"
             }
         },
+        "cxx2b": {
+            "label": "C++2b support",
+            "type": "compile",
+            "test": {
+                "head": [
+                    "#if __cplusplus > 202002L",
+                    "// Compiler claims to support experimental C++2b, trust it",
+                    "#else",
+                    "#  error __cplusplus must be > 202002L (the value for C++20)",
+                    "#endif"
+                ],
+                "qmake": "CONFIG += c++11 c++14 c++17 c++2a c++2b"
+            }
+        },
         "precompile_header": {
             "label": "precompiled header support",
             "type": "compile",
@@ -1002,6 +1016,12 @@
             "condition": "features.c++17 && tests.cxx2a",
             "output": [ "publicFeature", "publicQtConfig" ]
         },
+        "c++2b": {
+            "label": "C++2b",
+            "autoDetect": false,
+            "condition": "features.c++2a && tests.cxx2b",
+            "output": [ "publicFeature", "publicQtConfig" ]
+        },
         "c89": {
             "label": "C89"
         },
@@ -1554,7 +1574,7 @@ Configure with '-qreal float' to create a build that is binary-compatible with 5
                 {
                     "message": "Using C++ standard",
                     "type": "firstAvailableFeature",
-                    "args": "c++2a c++17 c++14 c++11"
+                    "args": "c++2b c++2a c++17 c++14 c++11"
                 },
                 {
                     "type": "feature",

configure.pri

@@ -22,18 +22,27 @@ defineTest(qtConfCommandline_cxxstd) {
             qtConfCommandlineSetInput("c++14", "no")
             qtConfCommandlineSetInput("c++1z", "no")
             qtConfCommandlineSetInput("c++2a", "no")
+            qtConfCommandlineSetInput("c++2b", "no")
         } else: contains(val, "(c\+\+)?(14|1y)") {
             qtConfCommandlineSetInput("c++14", "yes")
             qtConfCommandlineSetInput("c++1z", "no")
             qtConfCommandlineSetInput("c++2a", "no")
+            qtConfCommandlineSetInput("c++2b", "no")
         } else: contains(val, "(c\+\+)?(17|1z)") {
             qtConfCommandlineSetInput("c++14", "yes")
             qtConfCommandlineSetInput("c++1z", "yes")
             qtConfCommandlineSetInput("c++2a", "no")
+            qtConfCommandlineSetInput("c++2b", "no")
         } else: contains(val, "(c\+\+)?(2a)") {
             qtConfCommandlineSetInput("c++14", "yes")
             qtConfCommandlineSetInput("c++1z", "yes")
             qtConfCommandlineSetInput("c++2a", "yes")
+            qtConfCommandlineSetInput("c++2b", "no")
+        } else: contains(val, "(c\+\+)?(2b)") {
+            qtConfCommandlineSetInput("c++14", "yes")
+            qtConfCommandlineSetInput("c++1z", "yes")
+            qtConfCommandlineSetInput("c++2a", "yes")
+            qtConfCommandlineSetInput("c++2b", "yes")
         } else {
             qtConfAddError("Invalid argument $$val to command line parameter $$arg")
         }

mkspecs/common/clang.conf

@@ -35,10 +35,12 @@ QMAKE_CXXFLAGS_CXX11             = -std=c++11
 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_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_LFLAGS_CXX11      =
 QMAKE_LFLAGS_CXX14      =

mkspecs/common/g++-base.conf

@@ -33,10 +33,12 @@ QMAKE_CXXFLAGS_CXX11    = -std=c++11
 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_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_LFLAGS_CXX11      =
 QMAKE_LFLAGS_CXX14      =
 QMAKE_LFLAGS_CXX1Z      =

mkspecs/common/msvc-version.conf

@@ -118,6 +118,7 @@ greaterThan(QMAKE_MSC_VER, 1919) {
     # Visual Studio 2019 (16.0) / Visual C++ 19.20 and up
     MSVC_VER            = 16.0
     QMAKE_CXXFLAGS_CXX2A = -std:c++latest
+    QMAKE_CXXFLAGS_CXX2B = -std:c++latest
 
 }
 

mkspecs/features/default_post.prf

@@ -122,17 +122,18 @@ breakpad {
 }
 
 c++17: CONFIG += c++1z
-c++latest: CONFIG *= c++2a c++1z c++14 c++11
+c++latest: CONFIG *= c++2b c++2a c++1z c++14 c++11
 
-!c++11:!c++14:!c++1z:!c++2a {
+!c++11:!c++14:!c++1z:!c++2a:!c++2b {
     # Qt requires C++11 since 5.7, check if we need to force a compiler option
     QT_COMPILER_STDCXX_no_L = $$replace(QT_COMPILER_STDCXX, "L$", "")
     !greaterThan(QT_COMPILER_STDCXX_no_L, 199711): CONFIG += c++11
 }
-c++11|c++14|c++1z|c++2a {
+c++11|c++14|c++1z|c++2a|c++2b {
     # Disable special compiler flags for host builds
     !host_build|!cross_compile {
-        c++2a:       cxxstd = CXX2A
+        c++2b:       cxxstd = CXX2B
+        else: c++2a: cxxstd = CXX2A
         else: c++1z: cxxstd = CXX1Z
         else: c++14: cxxstd = CXX14
         else:        cxxstd = CXX11

mkspecs/features/qt_common.prf

@@ -18,6 +18,7 @@ qtConfig(c++11): CONFIG += c++11 strict_c++
 qtConfig(c++14): CONFIG += c++14
 qtConfig(c++1z): CONFIG += c++1z
 qtConfig(c++2a): CONFIG += c++2a
+qtConfig(c++2b): CONFIG += c++2b
 qtConfig(c99): CONFIG += c99
 qtConfig(c11): CONFIG += c11
 qtConfig(separate_debug_info): CONFIG += separate_debug_info
@@ -158,4 +159,3 @@ warnings_are_errors:warning_clean {
     }
     unset(ver)
 }
-

qmake/doc/src/qmake-manual.qdoc

@@ -1217,6 +1217,9 @@
     \row    \li c++2a \li C++2a support is enabled. This option has no effect if
             the compiler does not support C++2a, or can't select the C++ standard.
             By default, support is disabled.
+    \row    \li c++2b \li c++2b support is enabled. This option has no effect if
+            the compiler does not support c++2b, or can't select the C++ standard.
+            By default, support is disabled.
     \row    \li c++latest \li Support for the latest C++ language standard is
             enabled that is supported by the compiler. By default, this option is
             disabled.