Index: lld/COFF/PDB.cpp
===================================================================
--- lld/COFF/PDB.cpp
+++ lld/COFF/PDB.cpp
@@ -17,6 +17,7 @@
 #include "Writer.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Timer.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/DebugInfo/CodeView/DebugFrameDataSubsection.h"
 #include "llvm/DebugInfo/CodeView/DebugSubsectionRecord.h"
 #include "llvm/DebugInfo/CodeView/GlobalTypeTableBuilder.h"
@@ -226,9 +227,8 @@
   // decide that it's a unix path if we're fairly certain.  Specifically, if
   // it starts with a forward slash.
   SmallString<128> absoluteFileName = config->pdbSourcePath;
-  sys::path::Style guessedStyle = absoluteFileName.startswith("/")
-                                      ? sys::path::Style::posix
-                                      : sys::path::Style::windows;
+  sys::path::Style guessedStyle = sys::path::guess_style(absoluteFileName)
+                                      .getValueOr(sys::path::Style::native);
   sys::path::append(absoluteFileName, guessedStyle, fileName);
   sys::path::native(absoluteFileName, guessedStyle);
   sys::path::remove_dots(absoluteFileName, true, guessedStyle);
Index: lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
===================================================================
--- lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
+++ lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp
@@ -204,17 +204,17 @@
 
 static llvm::Optional<std::string>
 GetFileByIndex(const llvm::DWARFDebugLine::Prologue &prologue, size_t idx,
-               llvm::StringRef compile_dir, FileSpec::Style style) {
+               llvm::StringRef compile_dir) {
   // Try to get an absolute path first.
   std::string abs_path;
   auto absolute = llvm::DILineInfoSpecifier::FileLineInfoKind::AbsoluteFilePath;
-  if (prologue.getFileNameByIndex(idx, compile_dir, absolute, abs_path, style))
+  if (prologue.getFileNameByIndex(idx, compile_dir, absolute, abs_path))
     return std::move(abs_path);
 
   // Otherwise ask for a relative path.
   std::string rel_path;
   auto relative = llvm::DILineInfoSpecifier::FileLineInfoKind::RawValue;
-  if (!prologue.getFileNameByIndex(idx, compile_dir, relative, rel_path, style))
+  if (!prologue.getFileNameByIndex(idx, compile_dir, relative, rel_path))
     return {};
   return std::move(rel_path);
 }
@@ -237,7 +237,7 @@
   const size_t number_of_files = prologue.FileNames.size();
   for (size_t idx = first_file; idx <= number_of_files; ++idx) {
     std::string remapped_file;
-    if (auto file_path = GetFileByIndex(prologue, idx, compile_dir, style))
+    if (auto file_path = GetFileByIndex(prologue, idx, compile_dir))
       if (!module->RemapSourceFile(llvm::StringRef(*file_path), remapped_file))
         remapped_file = std::move(*file_path);
 
Index: llvm/include/llvm/DebugInfo/DWARF/DWARFDebugLine.h
===================================================================
--- llvm/include/llvm/DebugInfo/DWARF/DWARFDebugLine.h
+++ llvm/include/llvm/DebugInfo/DWARF/DWARFDebugLine.h
@@ -123,11 +123,9 @@
 
     Optional<uint64_t> getLastValidFileIndex() const;
 
-    bool
-    getFileNameByIndex(uint64_t FileIndex, StringRef CompDir,
-                       DILineInfoSpecifier::FileLineInfoKind Kind,
-                       std::string &Result,
-                       sys::path::Style Style = sys::path::Style::native) const;
+    bool getFileNameByIndex(uint64_t FileIndex, StringRef CompDir,
+                            DILineInfoSpecifier::FileLineInfoKind Kind,
+                            std::string &Result) const;
 
     void clear();
     void dump(raw_ostream &OS, DIDumpOptions DumpOptions) const;
Index: llvm/include/llvm/Support/Path.h
===================================================================
--- llvm/include/llvm/Support/Path.h
+++ llvm/include/llvm/Support/Path.h
@@ -476,6 +476,11 @@
 bool remove_dots(SmallVectorImpl<char> &path, bool remove_dot_dot = false,
                  Style style = Style::native);
 
+/// Attempt to guess path style for a given path string.
+///
+/// @param path Input path.
+llvm::Optional<Style> guess_style(StringRef absolute_path);
+
 } // end namespace path
 } // end namespace sys
 } // end namespace llvm
Index: llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp
===================================================================
--- llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp
+++ llvm/lib/DebugInfo/DWARF/DWARFDebugLine.cpp
@@ -1343,9 +1343,10 @@
          sys::path::is_absolute(Path, sys::path::Style::windows);
 }
 
-bool DWARFDebugLine::Prologue::getFileNameByIndex(
-    uint64_t FileIndex, StringRef CompDir, FileLineInfoKind Kind,
-    std::string &Result, sys::path::Style Style) const {
+bool DWARFDebugLine::Prologue::getFileNameByIndex(uint64_t FileIndex,
+                                                  StringRef CompDir,
+                                                  FileLineInfoKind Kind,
+                                                  std::string &Result) const {
   if (Kind == FileLineInfoKind::None || !hasFileAtIndex(FileIndex))
     return false;
   const FileNameEntry &Entry = getFileNameEntry(FileIndex);
@@ -1382,14 +1383,19 @@
   // We know that FileName is not absolute, the only way to have an absolute
   // path at this point would be if IncludeDir is absolute.
   if (Kind == FileLineInfoKind::AbsoluteFilePath && !CompDir.empty() &&
-      !isPathAbsoluteOnWindowsOrPosix(IncludeDir))
+      !isPathAbsoluteOnWindowsOrPosix(IncludeDir)) {
+    sys::path::Style Style =
+        sys::path::guess_style(CompDir).getValueOr(sys::path::Style::native);
     sys::path::append(FilePath, Style, CompDir);
+  }
 
   assert((Kind == FileLineInfoKind::AbsoluteFilePath ||
           Kind == FileLineInfoKind::RelativeFilePath) &&
          "invalid FileLineInfo Kind");
 
   // sys::path::append skips empty strings.
+  sys::path::Style Style =
+      sys::path::guess_style(IncludeDir).getValueOr(sys::path::Style::native);
   sys::path::append(FilePath, Style, IncludeDir, FileName);
   Result = std::string(FilePath.str());
   return true;
Index: llvm/lib/Support/Path.cpp
===================================================================
--- llvm/lib/Support/Path.cpp
+++ llvm/lib/Support/Path.cpp
@@ -763,6 +763,20 @@
   return true;
 }
 
+llvm::Optional<Style> guess_style(StringRef absolute_path) {
+  if (absolute_path.startswith("/"))
+    return Style::posix;
+  if (absolute_path.startswith(R"(\\)"))
+    return Style::windows;
+  if (absolute_path.size() > 3 && llvm::isAlpha(absolute_path[0]) &&
+      absolute_path.substr(1, 2) == R"(:\)")
+    return Style::windows;
+  const size_t n = absolute_path.find_first_of("/\\");
+  if (n != static_cast<size_t>(-1))
+    return absolute_path[n] == '/' ? Style::posix : Style::windows;
+  return llvm::None;
+}
+
 } // end namespace path
 
 namespace fs {
Index: llvm/lib/Support/VirtualFileSystem.cpp
===================================================================
--- llvm/lib/Support/VirtualFileSystem.cpp
+++ llvm/lib/Support/VirtualFileSystem.cpp
@@ -995,11 +995,8 @@
 /// Removes leading "./" as well as path components like ".." and ".".
 static llvm::SmallString<256> canonicalize(llvm::StringRef Path) {
   // First detect the path style in use by checking the first separator.
-  llvm::sys::path::Style style = llvm::sys::path::Style::native;
-  const size_t n = Path.find_first_of("/\\");
-  if (n != static_cast<size_t>(-1))
-    style = (Path[n] == '/') ? llvm::sys::path::Style::posix
-                             : llvm::sys::path::Style::windows;
+  llvm::sys::path::Style style = llvm::sys::path::guess_style(Path).getValueOr(
+      llvm::sys::path::Style::native);
 
   // Now remove the dots.  Explicitly specifying the path style prevents the
   // direction of the slashes from changing.
Index: llvm/unittests/Support/Path.cpp
===================================================================
--- llvm/unittests/Support/Path.cpp
+++ llvm/unittests/Support/Path.cpp
@@ -1521,6 +1521,17 @@
   EXPECT_EQ(Path, "C:\\old/foo\\bar");
 }
 
+TEST(Support, GuessStyle) {
+  EXPECT_EQ(path::Style::posix, path::guess_style("/foo/bar.txt"));
+  EXPECT_EQ(path::Style::posix, path::guess_style("//net/bar.txt"));
+  EXPECT_EQ(path::Style::windows,
+            path::guess_style(R"(C:\foo.txt)"));
+  EXPECT_EQ(path::Style::windows,
+            path::guess_style(R"(\\net\foo.txt)"));
+  EXPECT_EQ(llvm::None, path::guess_style("foo.txt"));
+  EXPECT_EQ(llvm::None, path::guess_style("foo/bar.txt"));
+}
+
 TEST_F(FileSystemTest, OpenFileForRead) {
   // Create a temp file.
   int FileDescriptor;