Index: clang/test/Index/index-module-with-vfs.m
===================================================================
--- clang/test/Index/index-module-with-vfs.m
+++ clang/test/Index/index-module-with-vfs.m
@@ -1,6 +1,3 @@
-// FIXME: PR43272
-// XFAIL: system-windows
-
 @import ModuleNeedsVFS;
 
 void foo() {
@@ -13,7 +10,7 @@
 // RUN: c-index-test -index-file %s -fmodules-cache-path=%t.cache -fmodules -F %t -I %t \
 // RUN:              -ivfsoverlay %t.yaml -Xclang -fdisable-module-hash | FileCheck %s
 
-// CHECK: [importedASTFile]: {{.*}}ModuleNeedsVFS.pcm | loc: 4:1 | name: "ModuleNeedsVFS" | isImplicit: 0
+// CHECK: [importedASTFile]: {{.*}}ModuleNeedsVFS.pcm | loc: 1:1 | name: "ModuleNeedsVFS" | isImplicit: 0
 // CHECK: [indexEntityReference]: kind: function | name: module_needs_vfs
 // CHECK: [indexEntityReference]: kind: function | name: base_module_needs_vfs
 
Index: clang/test/Modules/double-quotes.m
===================================================================
--- clang/test/Modules/double-quotes.m
+++ clang/test/Modules/double-quotes.m
@@ -1,6 +1,3 @@
-// FIXME: PR43272
-// XFAIL: system-windows
-
 // RUN: rm -rf %t
 // RUN: mkdir %t
 
Index: clang/test/Modules/framework-public-includes-private.m
===================================================================
--- clang/test/Modules/framework-public-includes-private.m
+++ clang/test/Modules/framework-public-includes-private.m
@@ -1,6 +1,3 @@
-// FIXME: PR43272
-// XFAIL: system-windows
-
 // RUN: rm -rf %t
 // RUN: mkdir %t
 
Index: clang/test/VFS/external-names.c
===================================================================
--- clang/test/VFS/external-names.c
+++ clang/test/VFS/external-names.c
@@ -1,9 +1,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" -e "s@EXTERNAL_NAMES@true@" %S/Inputs/use-external-names.yaml > %t.external.yaml
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" -e "s@EXTERNAL_NAMES@false@" %S/Inputs/use-external-names.yaml > %t.yaml
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "external-names.h"
 #ifdef REINCLUDE
 #include "external-names.h"
Index: clang/test/VFS/framework-import.m
===================================================================
--- clang/test/VFS/framework-import.m
+++ clang/test/VFS/framework-import.m
@@ -1,9 +1,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -F %t -ivfsoverlay %t.yaml -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #import <SomeFramework/public_header.h>
 
 void foo() {
Index: clang/test/VFS/implicit-include.c
===================================================================
--- clang/test/VFS/implicit-include.c
+++ clang/test/VFS/implicit-include.c
@@ -1,9 +1,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -ivfsoverlay %t.yaml -I %t -include "not_real.h" -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 void foo() {
   bar();
 }
Index: clang/test/VFS/include-mixed-real-and-virtual.c
===================================================================
--- clang/test/VFS/include-mixed-real-and-virtual.c
+++ clang/test/VFS/include-mixed-real-and-virtual.c
@@ -4,9 +4,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -ivfsoverlay %t.yaml -I %t -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "not_real.h"
 #include "real.h"
 
Index: clang/test/VFS/include-real-from-virtual.c
===================================================================
--- clang/test/VFS/include-real-from-virtual.c
+++ clang/test/VFS/include-real-from-virtual.c
@@ -4,9 +4,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -ivfsoverlay %t.yaml -I %t -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "include_real.h"
 
 void foo() {
Index: clang/test/VFS/include-virtual-from-real.c
===================================================================
--- clang/test/VFS/include-virtual-from-real.c
+++ clang/test/VFS/include-virtual-from-real.c
@@ -4,9 +4,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -ivfsoverlay %t.yaml -I %t -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "include_not_real.h"
 
 void foo() {
Index: clang/test/VFS/include.c
===================================================================
--- clang/test/VFS/include.c
+++ clang/test/VFS/include.c
@@ -1,9 +1,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -I %t -ivfsoverlay %t.yaml -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "not_real.h"
 
 void foo() {
Index: clang/test/VFS/incomplete-umbrella.m
===================================================================
--- clang/test/VFS/incomplete-umbrella.m
+++ clang/test/VFS/incomplete-umbrella.m
@@ -5,9 +5,6 @@
 // RUN: not %clang_cc1 -Werror -fmodules -fimplicit-module-maps -fmodules-cache-path=%t \
 // RUN:     -ivfsoverlay %t.yaml -F %t -fsyntax-only %s 2>&1 | FileCheck %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 @import Incomplete;
 // CHECK: umbrella header for module 'Incomplete' {{.*}}IncompleteVFS.h
 // CHECK: umbrella header for module 'Incomplete' {{.*}}IncompleteReal.h
Index: clang/test/VFS/module-import.m
===================================================================
--- clang/test/VFS/module-import.m
+++ clang/test/VFS/module-import.m
@@ -2,9 +2,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -ivfsoverlay %t.yaml -I %t -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 @import not_real;
 
 void foo() {
Index: clang/test/VFS/real-path-found-first.m
===================================================================
--- clang/test/VFS/real-path-found-first.m
+++ clang/test/VFS/real-path-found-first.m
@@ -4,9 +4,6 @@
 // intentionally rebuild modules, since the precompiled module file refers to
 // the dependency files by real path.
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 // RUN: rm -rf %t %t-cache %t.pch
 // RUN: mkdir -p %t/SomeFramework.framework/Modules
 // RUN: cat %S/Inputs/some_frame_module.map > %t/SomeFramework.framework/Modules/module.modulemap
Index: clang/test/VFS/relative-path.c
===================================================================
--- clang/test/VFS/relative-path.c
+++ clang/test/VFS/relative-path.c
@@ -3,9 +3,6 @@
 // RUN: sed -e "s@INPUT_DIR@%/S/Inputs@g" -e "s@OUT_DIR@%/t@g" %S/Inputs/vfsoverlay.yaml > %t.yaml
 // RUN: %clang_cc1 -Werror -I . -ivfsoverlay %t.yaml -fsyntax-only %s
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 #include "not_real.h"
 
 void foo() {
Index: clang/test/VFS/umbrella-framework-import-skipnonexist.m
===================================================================
--- clang/test/VFS/umbrella-framework-import-skipnonexist.m
+++ clang/test/VFS/umbrella-framework-import-skipnonexist.m
@@ -1,8 +1,5 @@
 // REQUIRES: crash-recovery
 
-// FIXME: PR43272
-// XFAIL: system-windows
-
 // RUN: rm -rf %t
 // RUN: mkdir -p %t/vdir %t/outdir %t/cache
 // RUN: cp -R %S/Inputs/Bar.framework %t/outdir/
Index: llvm/include/llvm/Support/VirtualFileSystem.h
===================================================================
--- llvm/include/llvm/Support/VirtualFileSystem.h
+++ llvm/include/llvm/Support/VirtualFileSystem.h
@@ -651,6 +651,15 @@
     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 to match backslashes (and vice versa).
+  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 +683,12 @@
   /// 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.
Index: llvm/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/lib/Support/VirtualFileSystem.cpp
+++ llvm/lib/Support/VirtualFileSystem.cpp
@@ -1671,9 +1671,7 @@
 
   // 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 @@
     if (Result || Result.getError() != llvm::errc::no_such_file_or_directory)
       return Result;
   }
+
   return make_error_code(llvm::errc::no_such_file_or_directory);
 }