Index: lld/test/ELF/lto/emit-llvm_assertion_failure.ll
===================================================================
--- /dev/null
+++ lld/test/ELF/lto/emit-llvm_assertion_failure.ll
@@ -0,0 +1,14 @@
+; 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.
+
+; REQUIRES: x86
+
+; RUN: opt -module-hash -module-summary %s -o %t.o
+; RUN: ld.lld --plugin-opt=emit-llvm -mllvm -bitcode-flush-threshold=0 -o /dev/null %t.o
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @main() {
+  ret void
+}
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
@@ -643,13 +643,14 @@
 
   // 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;
+  IsRegularFile = Status.type() == sys::fs::file_type::regular_file;
+#ifdef _WIN32
+  // MSVCRT's _lseek(SEEK_CUR) doesn't return -1 for pipes.
+  SupportsSeeking = !EC && IsRegularFile;
 #else
-  SupportsSeeking = loc != (off_t)-1;
+  SupportsSeeking = !EC && loc != (off_t)-1;
 #endif
   if (!SupportsSeeking)
     pos = 0;
@@ -914,8 +915,7 @@
   if (EC)
     return;
 
-  // Do not support non-seekable files.
-  if (!supportsSeeking())
+  if (!isRegularFile())
     EC = std::make_error_code(std::errc::invalid_argument);
 }