diff --git a/include/llvm/Support/VirtualFileSystem.h b/include/llvm/Support/VirtualFileSystem.h
index cdbfdbf0c4d..3b4c4aff9bd 100644
--- a/include/llvm/Support/VirtualFileSystem.h
+++ b/include/llvm/Support/VirtualFileSystem.h
@@ -532,7 +532,7 @@ class RedirectingFileSystemParser;
 /// \endverbatim
 ///
 /// All configuration options are optional.
-///   'case-sensitive': <boolean, default=true>
+///   'case-sensitive': <boolean, default=(true for Posix, false for Windows)>
 ///   'use-external-names': <boolean, default=true>
 ///   'overlay-relative': <boolean, default=false>
 ///   'fallthrough': <boolean, default=true>
@@ -651,6 +651,17 @@ private:
     return ExternalFSValidWD && IsFallthrough;
   }
 
+  // In a RedirectingFileSystem, keys can be specified in Posix or Windows
+  // style (or even a mixture of both), so this comparison helper allows
+  // slashes (representing a root) to match backslashes (and vice versa).  Note
+  // that, other than the root, patch components should not contain slashes or
+  // backslashes.
+  bool pathComponentMatches(llvm::StringRef lhs, llvm::StringRef rhs) const {
+    if ((CaseSensitive ? lhs.equals(rhs) : lhs.equals_lower(rhs)))
+      return true;
+    return (lhs == "/" && rhs == "\\") || (lhs == "\\" && rhs == "/");
+  }
+
   /// The root(s) of the virtual file system.
   std::vector<std::unique_ptr<Entry>> Roots;
 
@@ -674,7 +685,12 @@ private:
   /// Whether to perform case-sensitive comparisons.
   ///
   /// Currently, case-insensitive matching only works correctly with ASCII.
-  bool CaseSensitive = true;
+  bool CaseSensitive =
+#ifdef _WIN32
+      false;
+#else
+      true;
+#endif
 
   /// IsRelativeOverlay marks whether a ExternalContentsPrefixDir path must
   /// be prefixed in every 'external-contents' when reading from YAML files.
diff --git a/lib/Support/VirtualFileSystem.cpp b/lib/Support/VirtualFileSystem.cpp
index 40b748eab18..2d5c04baa57 100644
--- a/lib/Support/VirtualFileSystem.cpp
+++ b/lib/Support/VirtualFileSystem.cpp
@@ -1671,9 +1671,7 @@ RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
 
   // Forward the search to the next component in case this is an empty one.
   if (!FromName.empty()) {
-    if (CaseSensitive ? !Start->equals(FromName)
-                      : !Start->equals_lower(FromName))
-      // failure to match
+    if (!pathComponentMatches(*Start, FromName))
       return make_error_code(llvm::errc::no_such_file_or_directory);
 
     ++Start;
@@ -1695,6 +1693,7 @@ RedirectingFileSystem::lookupPath(sys::path::const_iterator Start,
     if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
       return Result;
   }
+
   return make_error_code(llvm::errc::no_such_file_or_directory);
 }