Index: lld/test/ELF/lto/emit-llvm.ll
===================================================================
--- lld/test/ELF/lto/emit-llvm.ll
+++ lld/test/ELF/lto/emit-llvm.ll
@@ -4,6 +4,10 @@
 ; RUN: ld.lld --plugin-opt=emit-llvm -o %t.out.o %t.o
 ; RUN: llvm-dis < %t.out.o -o - | FileCheck %s
 
+;; Make sure that if --plugin-opt=emit-llvm is enabled and the output
+;; is /dev/null, it should not cause a assertion failure in bitcode writer.
+; RUN: ld.lld --plugin-opt=emit-llvm -mllvm -bitcode-flush-threshold=0 -o /dev/null %t.o
+
 ; CHECK: define internal void @main()
 
 target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
Index: llvm/include/llvm/Support/raw_ostream.h
===================================================================
--- llvm/include/llvm/Support/raw_ostream.h
+++ llvm/include/llvm/Support/raw_ostream.h
@@ -444,6 +444,7 @@
   int FD;
   bool ShouldClose;
   bool SupportsSeeking = false;
+  bool IsRegularFile = false;
   mutable Optional<bool> HasColors;
 
 #ifdef _WIN32
@@ -514,6 +515,8 @@
 
   bool supportsSeeking() const { return SupportsSeeking; }
 
+  bool isRegularFile() const { return IsRegularFile; }
+
   /// Flushes the stream and repositions the underlying file descriptor position
   /// to the offset specified from the beginning of the file.
   uint64_t seek(uint64_t off);
Index: llvm/lib/Support/raw_ostream.cpp
===================================================================
--- llvm/lib/Support/raw_ostream.cpp
+++ llvm/lib/Support/raw_ostream.cpp
@@ -643,14 +643,10 @@
 
   // Get the starting position.
   off_t loc = ::lseek(FD, 0, SEEK_CUR);
-#ifdef _WIN32
-  // MSVCRT's _lseek(SEEK_CUR) doesn't return -1 for pipes.
   sys::fs::file_status Status;
   std::error_code EC = status(FD, Status);
-  SupportsSeeking = !EC && Status.type() == sys::fs::file_type::regular_file;
-#else
-  SupportsSeeking = loc != (off_t)-1;
-#endif
+  IsRegularFile = Status.type() == sys::fs::file_type::regular_file;
+  SupportsSeeking = !EC && IsRegularFile;
   if (!SupportsSeeking)
     pos = 0;
   else