diff --git a/clang/include/clang/Lex/DirectoryLookup.h b/clang/include/clang/Lex/DirectoryLookup.h
--- a/clang/include/clang/Lex/DirectoryLookup.h
+++ b/clang/include/clang/Lex/DirectoryLookup.h
@@ -91,6 +91,10 @@
     return isNormalDir() ? &u.Dir.getDirEntry() : nullptr;
   }
 
+  Optional<DirectoryEntryRef> getDirRef() const {
+    return isNormalDir() ? Optional<DirectoryEntryRef>(u.Dir) : None;
+  }
+
   /// getFrameworkDir - Return the directory that this framework refers to.
   ///
   const DirectoryEntry *getFrameworkDir() const {
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
@@ -436,10 +436,10 @@
   SmallString<1024> TmpDir;
   if (isNormalDir()) {
     // Concatenate the requested file onto the directory.
-    TmpDir = getDir()->getName();
+    TmpDir = getDirRef()->getName();
     llvm::sys::path::append(TmpDir, Filename);
     if (SearchPath) {
-      StringRef SearchPathRef(getDir()->getName());
+      StringRef SearchPathRef(getDirRef()->getName());
       SearchPath->clear();
       SearchPath->append(SearchPathRef.begin(), SearchPathRef.end());
     }
diff --git a/clang/test/ClangScanDeps/header-search-case-sensitivity.c b/clang/test/ClangScanDeps/header-search-case-sensitivity.c
new file mode 100644
--- /dev/null
+++ b/clang/test/ClangScanDeps/header-search-case-sensitivity.c
@@ -0,0 +1,50 @@
+// This test checks that reusing FileManager produces deterministic results on case-insensitive filesystems.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+//--- dir1/arm/lower.h
+//--- dir2/ARM/upper.h
+//--- t1.c
+#include "upper.h"
+//--- t2.c
+#include "arm/lower.h"
+
+//--- cdb.json.template
+[{
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/t1.c -I DIR/dir2/ARM -I DIR/dir1",
+  "file": "DIR/t1.c"
+},{
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/t2.c -I DIR/dir2     -I DIR/dir1",
+  "file": "DIR/t2.c"
+}]
+
+//--- cdb-rev.json.template
+[{
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/t2.c -I DIR/dir2     -I DIR/dir1",
+  "file": "DIR/t2.c"
+},{
+  "directory": "DIR",
+  "command": "clang -fsyntax-only DIR/t1.c -I DIR/dir2/ARM -I DIR/dir1",
+  "file": "DIR/t1.c"
+}]
+
+// RUN: sed -e "s|DIR|%/t|g" %t/cdb.json.template     > %t/cdb.json
+// RUN: sed -e "s|DIR|%/t|g" %t/cdb-rev.json.template > %t/cdb-rev.json
+
+// RUN: clang-scan-deps -compilation-database=%t/cdb.json     -format make -j 1 | sed 's:\\\\\?:/:g' | FileCheck %s
+
+// In the reversed case, Clang starts by scanning "t2.c". When looking up the "arm/lower.h" header,
+// the string is appended to "DIR/dir2". That file ("DIR/dir2/arm/lower.h") doesn't exist, but when
+// learning so, the FileManager stats and caches the parent directory ("DIR/dir2/arm"), using the
+// UID as the key.
+// When scanning "t1.c" later on, the "DIR/dir2/ARM" search directory is assigned the **same**
+// directory entry (with lowercase "arm"), since they share the UID on case-insensitive filesystems.
+// To preserve the correct case throughout the compiler for any file within that directory, it's
+// important to use the spelling actually used, not just the cached one.
+// RUN: clang-scan-deps -compilation-database=%t/cdb-rev.json -format make -j 1 | sed 's:\\\\\?:/:g' | FileCheck %s
+
+// CHECK: ARM/upper.h