1 files changed, 27 insertions, 4 deletions
diff --git a/clang/lib/Frontend/ModuleDependencyCollector.cpp b/clang/lib/Frontend/ModuleDependencyCollector.cpp
index d9cf40af0f6..1678d1801d2 100644
--- a/clang/lib/Frontend/ModuleDependencyCollector.cpp
+++ b/clang/lib/Frontend/ModuleDependencyCollector.cpp
@@ -79,19 +79,42 @@ void ModuleDependencyCollector::attachToPreprocessor(Preprocessor &PP) {
       llvm::make_unique<ModuleDependencyMMCallbacks>(*this));
 }
 
+static bool isCaseSensitivePath(StringRef Path) {
+  SmallString<PATH_MAX> TmpDest = Path, UpperDest, RealDest;
+  // Remove component traversals, links, etc.
+  if (!real_path(Path, TmpDest))
+    return true; // Current default value in vfs.yaml
+  Path = TmpDest;
+
+  // Change path to all upper case and ask for its real path, if the latter
+  // exists and is equal to Path, it's not case sensitive. Default to case
+  // sensitive in the absense of realpath, since this is what the VFSWriter
+  // already expects when sensitivity isn't setup.
+  for (auto &C : Path)
+    UpperDest.push_back(std::toupper(C));
+  if (real_path(UpperDest, RealDest) && Path.equals(RealDest))
+    return false;
+  return true;
+}
+
 void ModuleDependencyCollector::writeFileMap() {
   if (Seen.empty())
     return;
 
-  SmallString<256> Dest = getDest();
-  llvm::sys::path::append(Dest, "vfs.yaml");
+  StringRef VFSDir = getDest();
 
   // Default to use relative overlay directories in the VFS yaml file. This
   // allows crash reproducer scripts to work across machines.
-  VFSWriter.setOverlayDir(getDest());
+  VFSWriter.setOverlayDir(VFSDir);
+
+  // Explicitly set case sensitivity for the YAML writer. For that, find out
+  // the sensitivity at the path where the headers all collected to.
+  VFSWriter.setCaseSensitivity(isCaseSensitivePath(VFSDir));
 
   std::error_code EC;
-  llvm::raw_fd_ostream OS(Dest, EC, llvm::sys::fs::F_Text);
+  SmallString<256> YAMLPath = VFSDir;
+  llvm::sys::path::append(YAMLPath, "vfs.yaml");
+  llvm::raw_fd_ostream OS(YAMLPath, EC, llvm::sys::fs::F_Text);
   if (EC) {
     HasErrors = true;
     return;