diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1715,6 +1715,11 @@
         break;
       }
 
+      // Consider all path separators equal.
+      if (NI->size() == 1 && DI->size() == 1 &&
+          path::is_separator(NI->front()) && path::is_separator(DI->front()))
+        continue;
+
       if (*NI != *DI)
         break;
     }
diff --git a/clang/unittests/Lex/HeaderSearchTest.cpp b/clang/unittests/Lex/HeaderSearchTest.cpp
--- a/clang/unittests/Lex/HeaderSearchTest.cpp
+++ b/clang/unittests/Lex/HeaderSearchTest.cpp
@@ -98,6 +98,13 @@
                                                    /*WorkingDir=*/""),
             "z/t");
 }
+
+TEST_F(HeaderSearchTest, BackSlashWithDotDot) {
+  addSearchDir("..\\y");
+  EXPECT_EQ(Search.suggestPathToFileForDiagnostics("C:\\x\\y\\z\\t",
+                                                   /*WorkingDir=*/"C:/x/y/"),
+            "z/t");
+}
 #endif
 
 TEST_F(HeaderSearchTest, DotDotsWithAbsPath) {