Index: llvm/tools/llvm-rc/ResourceFileWriter.cpp
===================================================================
--- llvm/tools/llvm-rc/ResourceFileWriter.cpp
+++ llvm/tools/llvm-rc/ResourceFileWriter.cpp
@@ -1514,8 +1514,16 @@
   SmallString<128> Cwd;
   std::unique_ptr<MemoryBuffer> Result;
 
-  // 0. The file path is absolute and the file exists.
-  if (sys::path::is_absolute(File))
+  // 0. The file path is absolute or has a root directory, so we shouldn't
+  // try to append it on top of other base directories. (An absolute path
+  // must have a root directory, but e.g. the path "\dir\file" on windows
+  // isn't considered absolute, but it does have a root directory. As long as
+  // sys::path::append doesn't handle appending an absolute path or a path
+  // starting with a root directory on top of a base, we must handle this
+  // case separately at the top. C++17's path::append handles that case
+  // properly though, so if using that to append paths below, this early
+  // exception case could be removed.)
+  if (sys::path::has_root_directory(File))
     return errorOrToExpected(MemoryBuffer::getFile(File, -1, false));
 
   // 1. The current working directory.