Index: include/clang/Format/Format.h
===================================================================
--- include/clang/Format/Format.h
+++ include/clang/Format/Format.h
@@ -16,6 +16,7 @@
 #define LLVM_CLANG_FORMAT_FORMAT_H
 
 #include "clang/Basic/LangOptions.h"
+#include "clang/Basic/VirtualFileSystem.h"
 #include "clang/Tooling/Core/Replacement.h"
 #include "llvm/ADT/ArrayRef.h"
 #include <system_error>
@@ -832,11 +833,13 @@
 /// == "file".
 /// \param[in] FallbackStyle The name of a predefined style used to fallback to
 /// in case the style can't be determined from \p StyleName.
+/// \param[in] FS The underlying file system, in which the file resides. By
+/// default, the file system is the real file system.
 ///
 /// \returns FormatStyle as specified by ``StyleName``. If no style could be
 /// determined, the default is LLVM Style (see ``getLLVMStyle()``).
 FormatStyle getStyle(StringRef StyleName, StringRef FileName,
-                     StringRef FallbackStyle);
+                     StringRef FallbackStyle, vfs::FileSystem *FS = nullptr);
 
 } // end namespace format
 } // end namespace clang
Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -2098,7 +2098,10 @@
 }
 
 FormatStyle getStyle(StringRef StyleName, StringRef FileName,
-                     StringRef FallbackStyle) {
+                     StringRef FallbackStyle, vfs::FileSystem *FS) {
+  if (!FS) {
+    FS = vfs::getRealFileSystem().get();
+  }
   FormatStyle Style = getLLVMStyle();
   Style.Language = getLanguageByFileName(FileName);
   if (!getPredefinedStyle(FallbackStyle, Style.Language, &Style)) {
@@ -2129,28 +2132,35 @@
   llvm::sys::fs::make_absolute(Path);
   for (StringRef Directory = Path; !Directory.empty();
        Directory = llvm::sys::path::parent_path(Directory)) {
-    if (!llvm::sys::fs::is_directory(Directory))
+
+    auto Status = FS->status(Directory);
+    if (!Status ||
+        Status->getType() != llvm::sys::fs::file_type::directory_file) {
       continue;
+    }
+
     SmallString<128> ConfigFile(Directory);
 
     llvm::sys::path::append(ConfigFile, ".clang-format");
     DEBUG(llvm::dbgs() << "Trying " << ConfigFile << "...\n");
-    bool IsFile = false;
     // Ignore errors from is_regular_file: we only need to know if we can read
     // the file or not.
-    llvm::sys::fs::is_regular_file(Twine(ConfigFile), IsFile);
-
+    Status = FS->status(ConfigFile.str());
+    bool IsFile =
+        Status && (Status->getType() == llvm::sys::fs::file_type::regular_file);
     if (!IsFile) {
       // Try _clang-format too, since dotfiles are not commonly used on Windows.
       ConfigFile = Directory;
       llvm::sys::path::append(ConfigFile, "_clang-format");
       DEBUG(llvm::dbgs() << "Trying " << ConfigFile << "...\n");
-      llvm::sys::fs::is_regular_file(Twine(ConfigFile), IsFile);
+      Status = FS->status(ConfigFile.str());
+      IsFile = Status &&
+               (Status->getType() == llvm::sys::fs::file_type::regular_file);
     }
 
     if (IsFile) {
       llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
-          llvm::MemoryBuffer::getFile(ConfigFile.c_str());
+          FS->getBufferForFile(ConfigFile.str());
       if (std::error_code EC = Text.getError()) {
         llvm::errs() << EC.message() << "\n";
         break;
Index: unittests/Format/FormatTest.cpp
===================================================================
--- unittests/Format/FormatTest.cpp
+++ unittests/Format/FormatTest.cpp
@@ -14,6 +14,7 @@
 
 #include "clang/Frontend/TextDiagnosticPrinter.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/MemoryBuffer.h"
 #include "gtest/gtest.h"
 
 #define DEBUG_TYPE "format-test"
@@ -11199,6 +11200,33 @@
   verifyFormat("include \"a.td\"\ninclude \"b.td\"", Style);
 }
 
+TEST(FormatStyle, GetStyleOfFile) {
+  vfs::InMemoryFileSystem FS;
+  // Test 1: format file in the same directory.
+  ASSERT_TRUE(
+      FS.addFile("/a/.clang-format", 0,
+                 llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: LLVM")));
+  ASSERT_TRUE(
+      FS.addFile("/a/test.cpp", 0, llvm::MemoryBuffer::getMemBuffer("int i;")));
+  auto Style1 = getStyle("file", "/a/.clang-format", "Google", &FS);
+  ASSERT_EQ(Style1, getLLVMStyle());
+
+  // Test 2: fallback to default.
+  ASSERT_TRUE(
+      FS.addFile("/b/test.cpp", 0, llvm::MemoryBuffer::getMemBuffer("int i;")));
+  auto Style2 = getStyle("file", "/b/test.cpp", "Mozilla", &FS);
+  ASSERT_EQ(Style2, getMozillaStyle());
+
+  // Test 3: format file in parent directory.
+  ASSERT_TRUE(
+      FS.addFile("/c/.clang-format", 0,
+                 llvm::MemoryBuffer::getMemBuffer("BasedOnStyle: Google")));
+  ASSERT_TRUE(FS.addFile("/c/sub/sub/sub/test.cpp", 0,
+                         llvm::MemoryBuffer::getMemBuffer("int i;")));
+  auto Style3 = getStyle("file", "/c/sub/sub/sub/test.cpp", "LLVM", &FS);
+  ASSERT_EQ(Style3, getGoogleStyle());
+}
+
 class ReplacementTest : public ::testing::Test {
 protected:
   tooling::Replacement createReplacement(SourceLocation Start, unsigned Length,