[clang-format] Fix regression when getStyle() called with empty filename

Summary:
D43522 caused an assertion failure when getStyle() was called with
an empty filename:

P8065

This adds a test to reproduce the failure and fixes the issue by
ensuring we never pass an empty filename to
Environment::CreateVirtualEnvironment().

Test Plan: New test added. Ran test with:
  % make -j12 FormatTests && ./tools/clang/unittests/Format/FormatTests
  Before diff, test failed with P8065. Now, test passes.

Reviewers: vsapsai, jolesiak, krasimir

Reviewed By: vsapsai

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D43590

llvm-svn: 325722

1 files changed, 4 insertions, 3 deletions

diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 54b10f474c0..b82aab97113 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -2297,12 +2297,13 @@ static FormatStyle::LanguageKind getLanguageByFileName(StringRef FileName) {
 FormatStyle::LanguageKind guessLanguage(StringRef FileName, StringRef Code) {
   FormatStyle::LanguageKind result = getLanguageByFileName(FileName);
   if (result == FormatStyle::LK_Cpp) {
-    auto extension = llvm::sys::path::extension(FileName);
+    auto Extension = llvm::sys::path::extension(FileName);
     // If there's no file extension (or it's .h), we need to check the contents
     // of the code to see if it contains Objective-C.
-    if (extension.empty() || extension == ".h") {
+    if (Extension.empty() || Extension == ".h") {
+      auto NonEmptyFileName = FileName.empty() ? "guess.h" : FileName;
       std::unique_ptr<Environment> Env =
-          Environment::CreateVirtualEnvironment(Code, FileName, /*Ranges=*/{});
+          Environment::CreateVirtualEnvironment(Code, NonEmptyFileName, /*Ranges=*/{});
       ObjCHeaderStyleGuesser Guesser(*Env, getLLVMStyle());
       Guesser.process();
       if (Guesser.isObjC()) {