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/lib/Support/raw_ostream.cpp
===================================================================
--- llvm/lib/Support/raw_ostream.cpp
+++ llvm/lib/Support/raw_ostream.cpp
@@ -914,8 +914,9 @@
   if (EC)
     return;
 
-  // Do not support non-seekable files.
-  if (!supportsSeeking())
+  sys::fs::file_status Status;
+  EC = status(get_fd(), Status);
+  if (EC || Status.type() != sys::fs::file_type::regular_file)
     EC = std::make_error_code(std::errc::invalid_argument);
 }