msvc: fix passing /Zc:__cplusplus flag

It is mandatory to pass /Zc:__cplusplus when building
with Qt 6.2 and MSVC.

Fixes: QBS-1655
Change-Id: I27d928a568d0ac2075ce3f77f4142e20342be358
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

4 files changed, 33 insertions, 0 deletions

diff --git a/doc/reference/modules/cpp-module.qdoc b/doc/reference/modules/cpp-module.qdoc
index 1628bc80e..dba1d9d79 100644
--- a/doc/reference/modules/cpp-module.qdoc
+++ b/doc/reference/modules/cpp-module.qdoc
@@ -1925,3 +1925,17 @@
     \windowsproperty
     \nodefaultvalue
 */
+
+/*!
+    \qmlproperty string cpp::enableCxxLanguageMacro
+    \since Qbs 1.20
+
+    Whether to set the \c{/Zc:__cplusplus} macro
+    (\l{https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus})
+
+    This property only applies to MSVC toolchain. clang-cl also supports
+    this option, but it has no effect.
+
+    \windowsproperty
+    \defaultvalue \c{true} for MSVC, \c{false} for clang-cl
+*/
diff --git a/share/qbs/modules/cpp/msvc.js b/share/qbs/modules/cpp/msvc.js
index 1ebe5173b..3e528ddee 100644
--- a/share/qbs/modules/cpp/msvc.js
+++ b/share/qbs/modules/cpp/msvc.js
@@ -83,6 +83,19 @@ function hasCxx17Option(input)
             || (input.qbs.toolchain.contains("clang-cl") && input.cpp.compilerVersionMajor >= 7);
 }
 
+function hasZCplusPlusOption(input)
+{
+    // /Zc:__cplusplus is supported starting from Visual Studio 15.7
+    // Looks like closest MSVC version is 14.14.26428 (cl ver 19.14.26433)
+    // At least, this version is tested
+    // https://docs.microsoft.com/en-us/cpp/build/reference/zc-cplusplus
+    // clang-cl supports this option starting around-ish versions 8/9, but it
+    // ignores this option, so this doesn't really matter
+    // https://reviews.llvm.org/D45877
+    return Utilities.versionCompare(input.cpp.compilerVersion, "19.14.26433") >= 0
+            || (input.qbs.toolchain.contains("clang-cl") && input.cpp.compilerVersionMajor >= 9);
+}
+
 function supportsExternalIncludesOption(input) {
     if (input.qbs.toolchain.contains("clang-cl"))
         return false; // Exclude clang-cl.
@@ -240,6 +253,9 @@ function prepareCompiler(project, product, inputs, outputs, input, output, expli
     if (input.cpp.generateCompilerListingFiles)
         args.push("/Fa" + FileInfo.toWindowsSeparators(outputs.lst[0].filePath));
 
+    if (input.cpp.enableCxxLanguageMacro && hasZCplusPlusOption(input))
+        args.push("/Zc:__cplusplus");
+
     var objectMap = outputs.obj || outputs.intermediate_obj
     var objOutput = objectMap ? objectMap[0] : undefined
     args.push('/Fo' + FileInfo.toWindowsSeparators(objOutput.filePath))
diff --git a/share/qbs/modules/cpp/windows-msvc-base.qbs b/share/qbs/modules/cpp/windows-msvc-base.qbs
index 4efd20668..3d4975dda 100644
--- a/share/qbs/modules/cpp/windows-msvc-base.qbs
+++ b/share/qbs/modules/cpp/windows-msvc-base.qbs
@@ -104,6 +104,7 @@ CppModule {
     property var buildEnv
 
     readonly property bool shouldSignArtifacts: codesign.enableCodeSigning
+    property bool enableCxxLanguageMacro: false
 
     setupBuildEnvironment: {
         for (var key in product.cpp.buildEnv) {
diff --git a/share/qbs/modules/cpp/windows-msvc.qbs b/share/qbs/modules/cpp/windows-msvc.qbs
index 33c5e74c8..5e41c76fa 100644
--- a/share/qbs/modules/cpp/windows-msvc.qbs
+++ b/share/qbs/modules/cpp/windows-msvc.qbs
@@ -65,4 +65,6 @@ MsvcBaseModule {
     toolchainInstallPath: compilerPathProbe.found ? compilerPathProbe.path
                                                   : undefined
     buildEnv: msvcProbe.buildEnv
+
+    enableCxxLanguageMacro: true
 }