Detect the (stated) C++ standard edition at build time

The configure-time detection (cxx11default) isn't enough if the compiler
can be changed. This is especially necessary if Qt is compiled with a
compiler that defaults to >= C++11 (e.g., GCC 6) and then the user
selects a compiler another compiler (e.g., Clang) via -spec option. In
that case, we'd miss adding the -std=c++11 or -std=gnu++11 option to the
command-line, causing the compilation to fail.

As a nice side-effect, even moc without moc_predefs.h will now get the
__cplusplus setting.

Task-number: QTBUG-58321
Change-Id: I74966ed02f674a7295f8fffd14a8be35da9640e1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

config.tests/common/c++98default/c++98default.cpp

@@ -1,44 +0,0 @@
-/****************************************************************************
-**
-** Copyright (C) 2016 The Qt Company Ltd.
-** Contact: https://www.qt.io/licensing/
-**
-** This file is part of the configuration of the Qt Toolkit.
-**
-** $QT_BEGIN_LICENSE:LGPL$
-** Commercial License Usage
-** Licensees holding valid commercial Qt licenses may use this file in
-** accordance with the commercial license agreement provided with the
-** Software or, alternatively, in accordance with the terms contained in
-** a written agreement between you and The Qt Company. For licensing terms
-** and conditions see https://www.qt.io/terms-conditions. For further
-** information use the contact form at https://www.qt.io/contact-us.
-**
-** GNU Lesser General Public License Usage
-** Alternatively, this file may be used under the terms of the GNU Lesser
-** General Public License version 3 as published by the Free Software
-** Foundation and appearing in the file LICENSE.LGPL3 included in the
-** packaging of this file. Please review the following information to
-** ensure the GNU Lesser General Public License version 3 requirements
-** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
-**
-** GNU General Public License Usage
-** Alternatively, this file may be used under the terms of the GNU
-** General Public License version 2.0 or (at your option) the GNU General
-** Public license version 3 or any later version approved by the KDE Free
-** Qt Foundation. The licenses are as published by the Free Software
-** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
-** included in the packaging of this file. Please review the following
-** information to ensure the GNU General Public License requirements will
-** be met: https://www.gnu.org/licenses/gpl-2.0.html and
-** https://www.gnu.org/licenses/gpl-3.0.html.
-**
-** $QT_END_LICENSE$
-**
-****************************************************************************/
-
-#if __cplusplus < 201103L
-#error "compiler does not use c++11 or higher by default"
-#endif
-
-int main(int, char **) {}

config.tests/common/c++98default/c++98default.pro

@@ -1,3 +0,0 @@
-TEMPLATE = app
-CONFIG -= c++11 c++14 c++1z
-SOURCES = c++98default.cpp

configure.json

@@ -245,11 +245,6 @@
             "type": "compile",
             "test": "common/c++1z"
         },
-        "cxx11default": {
-            "label": "compiler defaulting to C++11 or higher",
-            "type": "compile",
-            "test": "common/c++98default"
-        },
         "precompile_header": {
             "label": "precompiled header support",
             "type": "compile",
@@ -451,11 +446,6 @@
             "condition": "call.crossCompile",
             "output": [ "publicConfig", "privateConfig", "crossCompile" ]
         },
-        "cxx11default": {
-            "label": "Compiler defaults to C++11 or higher",
-            "condition": "!tests.cxx11default",
-            "output": [ { "type": "publicConfig", "name": "c++11" } ]
-        },
         "compiler-flags": {
             "output": [ "compilerFlags" ]
         },

mkspecs/features/data/macros.cpp

@@ -1,4 +1,5 @@
 // Keep this file small. The pre-processed contents are eval'd by qmake.
+QT_COMPILER_STDCXX = __cplusplus
 #ifdef _MSC_VER
 QMAKE_MSC_VER = _MSC_VER
 QMAKE_MSC_FULL_VER = _MSC_FULL_VER

mkspecs/features/default_post.prf

@@ -95,6 +95,11 @@ breakpad {
     !isEmpty(QMAKE_STRIP):QMAKE_POST_LINK = $$QMAKE_POST_LINK$$escape_expand(\\n\\t)$$quote($$QMAKE_STRIP $$DEBUGFILENAME)
 }
 
+!c++11:!c++14:!c++1z {
+    # 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 {
     # Disable special compiler flags for host builds
     !host_build|!cross_compile {

mkspecs/features/toolchain.prf

@@ -137,6 +137,7 @@ unset(target_prefix)
 
 # Populate QMAKE_COMPILER_DEFINES and some compatibility variables.
 # The $$format_number() calls strip leading zeros to avoid misinterpretation as octal.
+QMAKE_COMPILER_DEFINES += __cplusplus=$$QT_COMPILER_STDCXX
 !isEmpty(QMAKE_MSC_VER) {
     QMAKE_COMPILER_DEFINES += _MSC_VER=$$QMAKE_MSC_VER _MSC_FULL_VER=$$QMAKE_MSC_FULL_VER
     QT_MSVC_MAJOR_VERSION = $$replace(QMAKE_MSC_FULL_VER, "(..)(..)(.*)", "\\1")