Index: lib/Basic/FileManager.cpp
===================================================================
--- lib/Basic/FileManager.cpp
+++ lib/Basic/FileManager.cpp
@@ -532,15 +532,17 @@
   if (Known != CanonicalDirNames.end())
     return Known->second;
 
-  StringRef CanonicalName(Dir->getName());
+  // Canonical name should still be an absolute path even if realpath fails.
+  llvm::SmallString<128> AbsDirPath(Dir->getName());
+  makeAbsolutePath(AbsDirPath);
+  StringRef CanonicalName = AbsDirPath.str().copy(CanonicalNameStorage);
 
 #ifdef LLVM_ON_UNIX
   char CanonicalNameBuf[PATH_MAX];
-  if (realpath(Dir->getName().str().c_str(), CanonicalNameBuf))
+  if (realpath(AbsDirPath.c_str(), CanonicalNameBuf))
     CanonicalName = StringRef(CanonicalNameBuf).copy(CanonicalNameStorage);
 #else
   SmallString<256> CanonicalNameBuf(CanonicalName);
-  llvm::sys::fs::make_absolute(CanonicalNameBuf);
   llvm::sys::path::native(CanonicalNameBuf);
   // We've run into needing to remove '..' here in the wild though, so
   // remove it.
Index: unittests/Basic/FileManagerTest.cpp
===================================================================
--- unittests/Basic/FileManagerTest.cpp
+++ unittests/Basic/FileManagerTest.cpp
@@ -322,4 +322,35 @@
   EXPECT_EQ(Path, ExpectedResult);
 }
 
+TEST_F(FileManagerTest, GetCanonicalNameReturnsAbsolutePathOnVFS) {
+  SmallString<64> CustomWorkingDir;
+#ifdef _WIN32
+  CustomWorkingDir = "C:\\test";
+#else
+  CustomWorkingDir = "/test";
+#endif
+  llvm::sys::path::append(CustomWorkingDir, "some", "weird", "path");
+
+  auto FS =
+      IntrusiveRefCntPtr<vfs::InMemoryFileSystem>(new vfs::InMemoryFileSystem);
+  // setCurrentworkingdirectory must finish without error.
+  ASSERT_TRUE(!FS->setCurrentWorkingDirectory(CustomWorkingDir));
+  SmallString<64> FooPath("weird/foo/foo.cpp");
+  FS->addFile(FooPath.str(), 0, llvm::MemoryBuffer::getMemBuffer(""));
+
+  FileSystemOptions Opts;
+  FileManager Manager(Opts, FS);
+
+  // Use relative path to open the directory.
+  const DirectoryEntry *Dir =
+      Manager.getDirectory(llvm::sys::path::parent_path(FooPath.str()));
+  EXPECT_TRUE(Dir != nullptr);
+  EXPECT_FALSE(llvm::sys::path::is_absolute(Dir->getName()));
+
+  SmallString<64> AbsFooPath(CustomWorkingDir);
+  llvm::sys::path::append(AbsFooPath, FooPath);
+  EXPECT_EQ(Manager.getCanonicalName(Dir),
+            llvm::sys::path::parent_path(AbsFooPath.str()));
+}
+
 } // anonymous namespace