diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1928,32 +1928,28 @@
     llvm::StringRef File, llvm::StringRef WorkingDir, llvm::StringRef MainFile,
     bool *IsSystem) {
   using namespace llvm::sys;
+  
+  llvm::SmallString<32> FilePath = File;
+  // remove_dots switches to backslashes on windows as a side-effect!
+  // We always want to suggest forward slashes for includes.
+  // (not remove_dots(..., posix) as that misparses windows paths).
+  path::remove_dots(FilePath, /*remove_dot_dot=*/true);
+  path::native(FilePath, path::Style::posix);
+  File = FilePath;
 
   unsigned BestPrefixLength = 0;
   // Checks whether `Dir` is a strict path prefix of `File`. If so and that's
   // the longest prefix we've seen so for it, returns true and updates the
   // `BestPrefixLength` accordingly.
-  auto CheckDir = [&](llvm::StringRef Dir) -> bool {
-    llvm::SmallString<32> DirPath(Dir.begin(), Dir.end());
+  auto CheckDir = [&](llvm::SmallString<32> Dir) -> bool {
     if (!WorkingDir.empty() && !path::is_absolute(Dir))
-      fs::make_absolute(WorkingDir, DirPath);
-    path::remove_dots(DirPath, /*remove_dot_dot=*/true);
-    Dir = DirPath;
+      fs::make_absolute(WorkingDir, Dir);
+    path::remove_dots(Dir, /*remove_dot_dot=*/true);
     for (auto NI = path::begin(File), NE = path::end(File),
               DI = path::begin(Dir), DE = path::end(Dir);
-         /*termination condition in loop*/; ++NI, ++DI) {
-      // '.' components in File are ignored.
-      while (NI != NE && *NI == ".")
-        ++NI;
-      if (NI == NE)
-        break;
-
-      // '.' components in Dir are ignored.
-      while (DI != DE && *DI == ".")
-        ++DI;
+         NI != NE; ++NI, ++DI) {
       if (DI == DE) {
-        // Dir is a prefix of File, up to '.' components and choice of path
-        // separators.
+        // Dir is a prefix of File, up to choice of path separators.
         unsigned PrefixLength = NI - path::begin(File);
         if (PrefixLength > BestPrefixLength) {
           BestPrefixLength = PrefixLength;
diff --git a/clang/unittests/Lex/HeaderSearchTest.cpp b/clang/unittests/Lex/HeaderSearchTest.cpp
--- a/clang/unittests/Lex/HeaderSearchTest.cpp
+++ b/clang/unittests/Lex/HeaderSearchTest.cpp
@@ -150,6 +150,14 @@
             "z");
 }
 
+TEST_F(HeaderSearchTest, BothDotDots) {
+  addSearchDir("/x/../y/");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/x/../y/z",
+                                                   /*WorkingDir=*/"",
+                                                   /*MainFile=*/""),
+            "z");
+}
+
 TEST_F(HeaderSearchTest, IncludeFromSameDirectory) {
   EXPECT_EQ(Search.suggestPathToFileForDiagnostics("/y/z/t.h",
                                                    /*WorkingDir=*/"",