diff --git a/clang/include/clang/Lex/HeaderSearch.h b/clang/include/clang/Lex/HeaderSearch.h
--- a/clang/include/clang/Lex/HeaderSearch.h
+++ b/clang/include/clang/Lex/HeaderSearch.h
@@ -727,6 +727,7 @@
   /// MainFile location, if none of the include search directories were prefix
   /// of File.
   ///
+  /// \param File A canonical absolute path to the file to be included.
   /// \param WorkingDir If non-empty, this will be prepended to search directory
   /// paths that are relative.
   std::string suggestPathToFileForDiagnostics(llvm::StringRef File,
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
@@ -1723,7 +1723,13 @@
     if (!WorkingDir.empty() && !path::is_absolute(Dir))
       fs::make_absolute(WorkingDir, DirPath);
     path::remove_dots(DirPath, /*remove_dot_dot=*/true);
-    Dir = DirPath;
+    // Find the canonical absolute path, as that's what we expect in File
+    auto& FS = FileMgr.getVirtualFileSystem();
+    SmallString<4096> CanonicalNameBuf;
+    if (!FS.getRealPath(DirPath, CanonicalNameBuf))
+      Dir = CanonicalNameBuf;
+    else
+      Dir = DirPath;
     for (auto NI = path::begin(File), NE = path::end(File),
               DI = path::begin(Dir), DE = path::end(Dir);
          /*termination condition in loop*/; ++NI, ++DI) {