diff --git a/clang/test/Lexer/case-insensitive-include-ms.c b/clang/test/Lexer/case-insensitive-include-ms.c
--- a/clang/test/Lexer/case-insensitive-include-ms.c
+++ b/clang/test/Lexer/case-insensitive-include-ms.c
@@ -6,15 +6,17 @@
 // RUN: %clang_cc1 -fsyntax-only -fms-compatibility %s -include %s -I %t/Output -verify
 // RUN: %clang_cc1 -fsyntax-only -fms-compatibility -fdiagnostics-parseable-fixits %s -include %s -I %t/Output 2>&1 | FileCheck %s
 
-// FIXME: Add a test with repeated backslashes once clang can handle that
-// in ms-compat mode on non-Windows hosts.
 #include "..\Output\.\case-insensitive-include.h"
 #include "..\Output\.\Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
 // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:50}:"\"..\\Output\\.\\case-insensitive-include.h\""
+#include "..\\Output\.\\Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:52}:"\"..\\\\Output\\.\\\\case-insensitive-include.h\""
 #include "..\output\.\case-insensitive-include.h" // expected-warning {{non-portable path}}
 // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:50}:"\"..\\Output\\.\\case-insensitive-include.h\""
 
 #include "apath\..\.\case-insensitive-include.h"
 #include "apath\..\.\Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
 // CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:49}:"\"apath\\..\\.\\case-insensitive-include.h\""
+#include "apath\\..\\.\\Case-Insensitive-Include.h" // expected-warning {{non-portable path}}
+// CHECK: fix-it:"{{.*}}":{[[@LINE-1]]:10-[[@LINE-1]]:52}:"\"apath\\\\..\\\\.\\\\case-insensitive-include.h\""
 #include "APath\..\.\case-insensitive-include.h" // For the sake of efficiency, this case is not diagnosed. :-(
diff --git a/llvm/lib/Support/Path.cpp b/llvm/lib/Support/Path.cpp
--- a/llvm/lib/Support/Path.cpp
+++ b/llvm/lib/Support/Path.cpp
@@ -540,15 +540,9 @@
       Path = PathHome;
     }
   } else {
-    for (auto PI = Path.begin(), PE = Path.end(); PI < PE; ++PI) {
-      if (*PI == '\\') {
-        auto PN = PI + 1;
-        if (PN < PE && *PN == '\\')
-          ++PI; // increment once, the for loop will move over the escaped slash
-        else
-          *PI = '/';
-      }
-    }
+    for (auto PI = Path.begin(), PE = Path.end(); PI < PE; ++PI)
+      if (*PI == '\\')
+        *PI = '/';
   }
 }
 
diff --git a/llvm/unittests/Support/Path.cpp b/llvm/unittests/Support/Path.cpp
--- a/llvm/unittests/Support/Path.cpp
+++ b/llvm/unittests/Support/Path.cpp
@@ -1182,7 +1182,7 @@
   Tests.emplace_back("a", "a", "a");
   Tests.emplace_back("a/b", "a\\b", "a/b");
   Tests.emplace_back("a\\b", "a\\b", "a/b");
-  Tests.emplace_back("a\\\\b", "a\\\\b", "a\\\\b");
+  Tests.emplace_back("a\\\\b", "a\\\\b", "a//b");
   Tests.emplace_back("\\a", "\\a", "/a");
   Tests.emplace_back("a\\", "a\\", "a/");
   Tests.emplace_back("a\\t", "a\\t", "a/t");