Index: lib/Basic/VirtualFileSystem.cpp
===================================================================
--- lib/Basic/VirtualFileSystem.cpp
+++ lib/Basic/VirtualFileSystem.cpp
@@ -1376,20 +1376,13 @@
   return UniqueID(std::numeric_limits<uint64_t>::max(), ID);
 }
 
-#ifndef NDEBUG
-static bool pathHasTraversal(StringRef Path) {
-  using namespace llvm::sys;
-  for (StringRef Comp : llvm::make_range(path::begin(Path), path::end(Path)))
-    if (Comp == "." || Comp == "..")
-      return true;
-  return false;
-}
-#endif
-
 void YAMLVFSWriter::addFileMapping(StringRef VirtualPath, StringRef RealPath) {
+  // Note that the path traversals "." or ".." aren't translated (removed)
+  // along the paths, but using the path string without translation is
+  // currently enough for VirtualPath and RealPath to work when reading out the
+  // YAML with the VFS description. Would it be cleaner to handle traversals?
   assert(sys::path::is_absolute(VirtualPath) && "virtual path not absolute");
   assert(sys::path::is_absolute(RealPath) && "real path not absolute");
-  assert(!pathHasTraversal(VirtualPath) && "path traversal is not supported");
   Mappings.emplace_back(VirtualPath, RealPath);
 }
 
Index: test/Modules/crash-vfs-path-traversal.m
===================================================================
--- /dev/null
+++ test/Modules/crash-vfs-path-traversal.m
@@ -0,0 +1,44 @@
+// REQUIRES: crash-recovery, shell
+
+// FIXME: This XFAIL is cargo-culted from crash-report.c. Do we need it?
+// XFAIL: mingw32
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t/i %t/m %t
+
+// RUN: not env FORCE_CLANG_DIAGNOSTICS_CRASH= TMPDIR=%t TEMP=%t TMP=%t \
+// RUN: %clang -fsyntax-only %s -I %S/Inputs/System -isysroot %/t/i/    \
+// RUN: -fmodules -fmodules-cache-path=%t/m/ 2>&1 | FileCheck %s
+
+// RUN: FileCheck --check-prefix=CHECKSRC %s -input-file %t/crash-vfs-*.m
+// RUN: FileCheck --check-prefix=CHECKSH %s -input-file %t/crash-vfs-*.sh
+// RUN: FileCheck --check-prefix=CHECKYAML %s -input-file \
+// RUN: %t/crash-vfs-*.cache/vfs/vfs.yaml
+// RUN: find %t/crash-vfs-*.cache/vfs | \
+// RUN: 	grep "Inputs/System/usr/include/stdio.h" | count 1
+
+#include "usr/include/../include/stdio.h"
+
+// CHECK: Preprocessed source(s) and associated run script(s) are located at:
+// CHECK-NEXT: note: diagnostic msg: {{.*}}.m
+// CHECK-NEXT: note: diagnostic msg: {{.*}}.cache
+
+// CHECKSRC: @import cstd.stdio;
+
+// CHECKSH: # Crash reproducer
+// CHECKSH-NEXT: # Driver args: "-fsyntax-only"
+// CHECKSH-NEXT: # Original command: {{.*$}}
+// CHECKSH-NEXT: "-cc1"
+// CHECKSH: "-isysroot" "{{[^"]*}}/i/"
+// CHECKSH-NOT: "-fmodules-cache-path="
+// CHECKSH: "crash-vfs-{{[^ ]*}}.m"
+// CHECKSH: "-ivfsoverlay" "crash-vfs-{{[^ ]*}}.cache/vfs/vfs.yaml"
+
+// CHECKYAML: 'type': 'directory'
+// CHECKYAML: 'name': "{{[^ ]*}}/Inputs/System/usr/include/../include",
+// CHECKYAML-NEXT: 'contents': [
+// CHECKYAML-NEXT:   {
+// CHECKYAML-NEXT:     'type': 'file',
+// CHECKYAML-NEXT:     'name': "module.map",
+// CHECKYAML-NEXT:     'external-contents': "{{[^ ]*}}/Inputs/System/usr/include/../include/module.map"
+// CHECKYAML-NEXT:   },