Index: include/clang/Basic/VirtualFileSystem.h
===================================================================
--- include/clang/Basic/VirtualFileSystem.h
+++ include/clang/Basic/VirtualFileSystem.h
@@ -374,6 +374,14 @@
   llvm::ErrorOr<std::string> getCurrentWorkingDirectory() const override {
     return WorkingDirectory;
   }
+  /// Canonicalizes \p Path by combining with the current working
+  /// directory and normalizing the path (e.g. remove dots). If the current
+  /// working directory is not set, this returns errc::operation_not_permitted.
+  ///
+  /// This doesn't resolve symlinks as they are not supported in in-memory file
+  /// system.
+  std::error_code getRealPath(const Twine &Path,
+                              SmallVectorImpl<char> &Output) const override;
 
   std::error_code setCurrentWorkingDirectory(const Twine &Path) override;
 };
Index: lib/Basic/VirtualFileSystem.cpp
===================================================================
--- lib/Basic/VirtualFileSystem.cpp
+++ lib/Basic/VirtualFileSystem.cpp
@@ -788,6 +788,19 @@
   return {};
 }
 
+std::error_code
+InMemoryFileSystem::getRealPath(const Twine &Path,
+                                SmallVectorImpl<char> &Output) const {
+  auto CWD = getCurrentWorkingDirectory();
+  if (!CWD || CWD->empty())
+    return errc::operation_not_permitted;
+  Path.toVector(Output);
+  if (auto EC = makeAbsolute(Output))
+    return EC;
+  llvm::sys::path::remove_dots(Output, /*remove_dot_dot=*/true);
+  return {};
+}
+
 } // namespace vfs
 } // namespace clang
 
Index: unittests/Basic/VirtualFileSystemTest.cpp
===================================================================
--- unittests/Basic/VirtualFileSystemTest.cpp
+++ unittests/Basic/VirtualFileSystemTest.cpp
@@ -813,6 +813,28 @@
                       NormalizedFS.getCurrentWorkingDirectory().get()));
 }
 
+TEST_F(InMemoryFileSystemTest, GetRealPath) {
+  SmallString<16> Path;
+  EXPECT_EQ(FS.getRealPath("b", Path), errc::operation_not_permitted);
+
+  auto GetRealPath = [this](StringRef P) {
+    SmallString<16> Output;
+    auto EC = FS.getRealPath(P, Output);
+    EXPECT_FALSE(EC);
+    return Output.str().str();
+  };
+
+  FS.setCurrentWorkingDirectory("a");
+  EXPECT_EQ(GetRealPath("b"), "a/b");
+  EXPECT_EQ(GetRealPath("../b"), "b");
+  EXPECT_EQ(GetRealPath("b/./c"), "a/b/c");
+
+  FS.setCurrentWorkingDirectory("/a");
+  EXPECT_EQ(GetRealPath("b"), "/a/b");
+  EXPECT_EQ(GetRealPath("../b"), "/b");
+  EXPECT_EQ(GetRealPath("b/./c"), "/a/b/c");
+}
+
 TEST_F(InMemoryFileSystemTest, AddFileWithUser) {
   FS.addFile("/a/b/c", 0, MemoryBuffer::getMemBuffer("abc"), 0xFEEDFACE);
   auto Stat = FS.status("/a");