Merge topic 'clangcl-fixes'

091afa7342 Tests: Teach tests when to treat clang-cl as MSVC
006768903c Work around clang-cl breakage on make_unique/std::forward
d625dfcdf9 Avoid compiling CMake itself as C++17 with Clang's MSVC ABI

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !2573

Source/Checks/cm_cxx17_check.cpp

@@ -2,8 +2,21 @@
 #include <memory>
 #include <unordered_map>
 
+#ifdef _MSC_VER
+#  include <comdef.h>
+#endif
+
 int main()
 {
   std::unique_ptr<int> u(new int(0));
+
+#ifdef _MSC_VER
+  // clang-cl has problems instantiating this constructor in C++17 mode
+  //  error: indirection requires pointer operand ('const _GUID' invalid)
+  //      return *_IID;
+  IUnknownPtr ptr{};
+  IDispatchPtr disp(ptr);
+#endif
+
   return *u;
 }

Source/cmExportFileGenerator.cxx

@@ -70,8 +70,9 @@ bool cmExportFileGenerator::GenerateImportFile()
   std::unique_ptr<cmsys::ofstream> foutPtr;
   if (this->AppendMode) {
     // Open for append.
+    auto openmodeApp = std::ios::app;
     foutPtr = cm::make_unique<cmsys::ofstream>(this->MainImportFile.c_str(),
-                                               std::ios::app);
+                                               openmodeApp);
   } else {
     // Generate atomically and with copy-if-different.
     std::unique_ptr<cmGeneratedFileStream> ap(

Source/cmExportLibraryDependenciesCommand.cxx

@@ -50,8 +50,9 @@ void cmExportLibraryDependenciesCommand::ConstFinalPass() const
   // Use copy-if-different if not appending.
   std::unique_ptr<cmsys::ofstream> foutPtr;
   if (this->Append) {
+    const auto openmodeApp = std::ios::app;
     foutPtr =
-      cm::make_unique<cmsys::ofstream>(this->Filename.c_str(), std::ios::app);
+      cm::make_unique<cmsys::ofstream>(this->Filename.c_str(), openmodeApp);
   } else {
     std::unique_ptr<cmGeneratedFileStream> ap(
       new cmGeneratedFileStream(this->Filename, true));