MSVC: Use "/external:I" flag to set the system include paths

Previously we were using the "/I" compiler option to specify
the system include paths, because the MSVC compiler had not
the special option for that.

But since v15.6, the MSVC 2017 compiler has the special
"/external:I" option to specify the system include paths:

 * https://devblogs.microsoft.com/cppblog/broken-warnings-theory/

So, it makes sense to use this new option for the MSVC versions
which are supports it (since _MSC_VER 1913, aka MSVC 2017
update 6, v15. 6.0):

 * https://dev.to/yumetodo/list-of-mscver-and-mscfullver-8nd

But, due to MSVC 2017 update 6 has some bugs:

 * https://developercommunity.visualstudio.com/content/problem/181006/externali-include-paths-not-working.html

which are fixed since MSVC 2017 update 9 (aka _MSC_VER 1916),
then we just keep _MSC_VER 1916 as a base version.

In other cases we fall back to the "/I" option.

Fixes: QBS-1573
Change-Id: Ie180ba61a1988aaec7955f545b989edf406c7730
Reviewed-by: Ivan Komissarov <ABBAPOH@gmail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

1 files changed, 26 insertions, 6 deletions

diff --git a/share/qbs/modules/cpp/msvc.js b/share/qbs/modules/cpp/msvc.js
index 8611ebe50..53f386996 100644
--- a/share/qbs/modules/cpp/msvc.js
+++ b/share/qbs/modules/cpp/msvc.js
@@ -82,6 +82,16 @@ function hasCxx17Option(input)
             || (input.qbs.toolchain.contains("clang-cl") && input.cpp.compilerVersionMajor >= 7);
 }
 
+function supportsExternalIncludesOption(input) {
+    if (input.qbs.toolchain.contains("clang-cl"))
+        return false; // Exclude clang-cl.
+    // This option was introcuded since MSVC 2017 v15.6 (aka _MSC_VER 19.13).
+    // But due to some MSVC bugs:
+    // * https://developercommunity.visualstudio.com/content/problem/181006/externali-include-paths-not-working.html
+    // this option has been fixed since MSVC 2017 update 9, v15.9 (aka _MSC_VER 19.16).
+    return Utilities.versionCompare(input.cpp.compilerVersion, "19.16") >= 0;
+}
+
 function addLanguageVersionFlag(input, args) {
     var cxxVersion = Cpp.languageVersion(input.cpp.cxxLanguageVersion,
                                          ["c++17", "c++14", "c++11", "c++98"], "C++");
@@ -189,15 +199,25 @@ function prepareCompiler(project, product, inputs, outputs, input, output, expli
         args.push('/Wall')
     if (input.cpp.treatWarningsAsErrors)
         args.push('/WX')
-    var allIncludePaths = [];
     var includePaths = input.cpp.includePaths;
-    if (includePaths)
-        allIncludePaths = allIncludePaths.uniqueConcat(includePaths);
+    if (includePaths) {
+        args = args.concat([].uniqueConcat(includePaths).map(function(path) {
+            return '/I' + FileInfo.toWindowsSeparators(path);
+        }));
+    }
+
+    var allSystemIncludePaths = [];
     var systemIncludePaths = input.cpp.systemIncludePaths;
     if (systemIncludePaths)
-        allIncludePaths = allIncludePaths.uniqueConcat(systemIncludePaths);
-    for (i in allIncludePaths)
-        args.push('/I' + FileInfo.toWindowsSeparators(allIncludePaths[i]))
+        allSystemIncludePaths = allSystemIncludePaths.uniqueConcat(systemIncludePaths);
+    var includeFlag = "/I";
+    if (supportsExternalIncludesOption(input)) {
+        args.push("/experimental:external");
+        includeFlag = "/external:I"
+    }
+    allSystemIncludePaths.forEach(function(path) {
+        args.push(includeFlag, FileInfo.toWindowsSeparators(path)); });
+
     var allDefines = [];
     var platformDefines = input.cpp.platformDefines;
     if (platformDefines)