diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 660aba1db7af..3b7eaee3c914 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -1785,20 +1785,23 @@ Optional<FileEntryRef> Preprocessor::LookupHeaderIncludeOrImport(
       return Filename;
     };
     StringRef TypoCorrectionName = CorrectTypoFilename(Filename);
-    SmallString<128> NormalizedTypoCorrectionPath;
-    if (LangOpts.MSVCCompat) {
-      NormalizedTypoCorrectionPath = TypoCorrectionName.str();
+
 #ifndef _WIN32
+    // Normalize slashes when compiling with -fms-extensions on non-Windows.
+    // This is unnecessary on Windows since the filesystem there handles
+    // backslashes.
+    SmallString<128> NormalizedTypoCorrectionPath;
+    if (LangOpts.MicrosoftExt) {
+      NormalizedTypoCorrectionPath = TypoCorrectionName;
       llvm::sys::path::native(NormalizedTypoCorrectionPath);
-#endif
+      TypoCorrectionName = NormalizedTypoCorrectionPath;
     }
+#endif
+
     Optional<FileEntryRef> File = LookupFile(
-        FilenameLoc,
-        LangOpts.MSVCCompat ? NormalizedTypoCorrectionPath.c_str()
-                            : TypoCorrectionName,
-        isAngled, LookupFrom, LookupFromFile, CurDir,
-        Callbacks ? &SearchPath : nullptr, Callbacks ? &RelativePath : nullptr,
-        &SuggestedModule, &IsMapped,
+        FilenameLoc, TypoCorrectionName, isAngled, LookupFrom, LookupFromFile,
+        CurDir, Callbacks ? &SearchPath : nullptr,
+        Callbacks ? &RelativePath : nullptr, &SuggestedModule, &IsMapped,
         /*IsFrameworkFound=*/nullptr);
     if (File) {
       auto Hint =
@@ -1906,15 +1909,18 @@ Preprocessor::ImportAction Preprocessor::HandleHeaderIncludeOrImport(
   // the path.
   ModuleMap::KnownHeader SuggestedModule;
   SourceLocation FilenameLoc = FilenameTok.getLocation();
-  SmallString<128> NormalizedPath;
-  if (LangOpts.MSVCCompat) {
-    NormalizedPath = Filename.str();
+  StringRef LookupFilename = Filename;
+
 #ifndef _WIN32
+  // Normalize slashes when compiling with -fms-extensions on non-Windows. This
+  // is unnecessary on Windows since the filesystem there handles backslashes.
+  SmallString<128> NormalizedPath;
+  if (LangOpts.MicrosoftExt) {
+    NormalizedPath = Filename.str();
     llvm::sys::path::native(NormalizedPath);
-#endif
+    LookupFilename = NormalizedPath;
   }
-  StringRef LookupFilename =
-      LangOpts.MSVCCompat ? StringRef(NormalizedPath) : Filename;
+#endif
 
   Optional<FileEntryRef> File = LookupHeaderIncludeOrImport(
       CurDir, Filename, FilenameLoc, FilenameRange, FilenameTok,
diff --git a/clang/test/Lexer/cross-windows-on-linux-default.cpp b/clang/test/Lexer/cross-windows-on-linux-default.cpp
index 520b419a7100..bcd4999a663a 100644
--- a/clang/test/Lexer/cross-windows-on-linux-default.cpp
+++ b/clang/test/Lexer/cross-windows-on-linux-default.cpp
@@ -1,4 +1,4 @@
-// RUN: not %clang_cc1 -fsyntax-only -fms-compatibility -triple i686-win32 %s 2>&1 \
+// RUN: not %clang_cc1 -fsyntax-only -fms-extensions -triple i686-win32 %s 2>&1 \
 // RUN:   | FileCheck %s
 
 #include "Inputs\success.h"
diff --git a/clang/test/Lexer/cross-windows-on-linux.cpp b/clang/test/Lexer/cross-windows-on-linux.cpp
index c6dcbca54056..3932ffcb33c8 100644
--- a/clang/test/Lexer/cross-windows-on-linux.cpp
+++ b/clang/test/Lexer/cross-windows-on-linux.cpp
@@ -6,10 +6,8 @@
 // CHECK: #include "Inputs\success.h"
 // CHECK:          ^
 
-// expected to fail on windows as the inclusion would succeed and the
-// compilation will fail due to the '#error success'.
-// XFAIL: windows-msvc
-
-// This test may or may not fail since 'Inputs\success.h' is passed
-// to Win32 APIs on Windows.
-// REQUIRES: disabled
+// This test is really checking that we *don't* replace backslashes with slashes
+// on non-Windows unless -fms-extensions is passed. It won't fail in this way on
+// Windows because the filesystem will interpret the backslash as a directory
+// separator.
+// UNSUPPORTED: system-windows