Index: lib/Basic/SourceManager.cpp
===================================================================
--- lib/Basic/SourceManager.cpp
+++ lib/Basic/SourceManager.cpp
@@ -2219,10 +2219,11 @@
   // is deleted.
   IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem> InMemoryFileSystem(
       new llvm::vfs::InMemoryFileSystem);
-  InMemoryFileSystem->addFile(
+  bool DidAddFile = InMemoryFileSystem->addFile(
       FileName, 0,
       llvm::MemoryBuffer::getMemBuffer(Content, FileName,
                                        /*RequiresNullTerminator=*/false));
+  assert(DidAddFile && "Could not add file to InMemoryFileSystem");
   // This is passed to `SM` as reference, so the pointer has to be referenced
   // in `Environment` so that `FileMgr` can out-live this function scope.
   FileMgr =
Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -1984,6 +1984,7 @@
 tooling::Replacements sortIncludes(const FormatStyle &Style, StringRef Code,
                                    ArrayRef<tooling::Range> Ranges,
                                    StringRef FileName, unsigned *Cursor) {
+  assert(!FileName.empty());
   tooling::Replacements Replaces;
   if (!Style.SortIncludes)
     return Replaces;
@@ -2152,6 +2153,7 @@
          ArrayRef<tooling::Range> Ranges, unsigned FirstStartColumn,
          unsigned NextStartColumn, unsigned LastStartColumn, StringRef FileName,
          FormattingAttemptStatus *Status) {
+  assert(!FileName.empty());
   FormatStyle Expanded = expandPresets(Style);
   if (Expanded.DisableFormat)
     return {tooling::Replacements(), 0};
@@ -2228,6 +2230,7 @@
 tooling::Replacements cleanup(const FormatStyle &Style, StringRef Code,
                               ArrayRef<tooling::Range> Ranges,
                               StringRef FileName) {
+  assert(!FileName.empty());
   // cleanups only apply to C++ (they mostly concern ctor commas etc.)
   if (Style.Language != FormatStyle::LK_Cpp)
     return tooling::Replacements();
@@ -2248,6 +2251,7 @@
                                               StringRef Code,
                                               ArrayRef<tooling::Range> Ranges,
                                               StringRef FileName) {
+  assert(!FileName.empty());
   return NamespaceEndCommentsFixer(Environment(Code, FileName, Ranges), Style)
       .process()
       .first;
@@ -2257,6 +2261,7 @@
                                             StringRef Code,
                                             ArrayRef<tooling::Range> Ranges,
                                             StringRef FileName) {
+  assert(!FileName.empty());
   return UsingDeclarationsSorter(Environment(Code, FileName, Ranges), Style)
       .process()
       .first;
Index: tools/clang-format/ClangFormat.cpp
===================================================================
--- tools/clang-format/ClangFormat.cpp
+++ tools/clang-format/ClangFormat.cpp
@@ -117,7 +117,8 @@
 static FileID createInMemoryFile(StringRef FileName, MemoryBuffer *Source,
                                  SourceManager &Sources, FileManager &Files,
                                  llvm::vfs::InMemoryFileSystem *MemFS) {
-  MemFS->addFileNoOwn(FileName, 0, Source);
+  bool DidAddFile = MemFS->addFileNoOwn(FileName, 0, Source);
+  assert(DidAddFile && "Could not add file to InMemoryFileSystem");
   return Sources.createFileID(Files.getFile(FileName), SourceLocation(),
                               SrcMgr::C_User);
 }
@@ -262,6 +263,10 @@
   if (fillRanges(Code.get(), Ranges))
     return true;
   StringRef AssumedFileName = (FileName == "-") ? AssumeFileName : FileName;
+  if (AssumedFileName.empty()) {
+    llvm::errs() << "error: empty filenames are not allowed\n";
+    return true;
+  }
 
   llvm::Expected<FormatStyle> FormatStyle =
       getStyle(Style, AssumedFileName, FallbackStyle, Code->getBuffer());