Index: lib/Core/Reproduce.cpp
===================================================================
--- lib/Core/Reproduce.cpp
+++ lib/Core/Reproduce.cpp
@@ -76,6 +76,8 @@
 // Makes a given pathname an absolute path first, and then remove
 // beginning /. For example, "../foo.o" is converted to "home/john/foo.o",
 // assuming that the current directory is "/home/john/bar".
+// Returned string is a backslash-separated path even on Windows to avoid
+// a mess with backslash-as-escape and backslash-as-path-separator.
 std::string lld::relativeToRoot(StringRef Path) {
   SmallString<128> Abs = Path;
   if (sys::fs::make_absolute(Abs))
@@ -93,6 +95,7 @@
     Res = Root.substr(2);
 
   path::append(Res, path::relative_path(Abs));
+  std::replace(Res.begin(), Res.end(), '\\', '/');
 
   return Res.str();
 }
Index: test/ELF/reproduce-windows2.s
===================================================================
--- test/ELF/reproduce-windows2.s
+++ test/ELF/reproduce-windows2.s
@@ -0,0 +1,11 @@
+# REQUIRES: windows, x86, cpio
+
+# Test that a response.txt file always uses / instead of \.
+# RUN: rm -rf %t.dir
+# RUN: mkdir -p %t.dir/build
+# RUN: llvm-mc %s -o %t.dir/build/foo.o -filetype=obj -triple=x86_64-pc-linux
+# RUN: cd %t.dir
+# RUN: ld.lld build/foo.o --reproduce repro
+# RUN: echo "*response.txt" > list.txt
+# RUN: cpio -i --to-stdout --pattern-file=list.txt < repro.cpio | FileCheck %s
+# CHECK: {{.*}}/build/foo.o
Index: test/lit.cfg
===================================================================
--- test/lit.cfg
+++ test/lit.cfg
@@ -207,6 +207,10 @@
 if platform.system() in ['FreeBSD', 'Linux']:
     config.available_features.add('system-linker-elf')
 
+# Running on Windows
+if platform.system() in ['Windows']:
+    config.available_features.add('windows')
+
 # Set if host-cxxabi's demangler can handle target's symbols.
 if platform.system() not in ['Windows']:
     config.available_features.add('demangler')