diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -4588,12 +4588,6 @@
                              Res.getPreprocessorOpts().Includes, Diags);
     if (Res.getFrontendOpts().ProgramAction == frontend::RewriteObjC)
       LangOpts.ObjCExceptions = 1;
-    if (T.isOSDarwin() && DashX.isPreprocessed()) {
-      // Supress the darwin-specific 'stdlibcxx-not-found' diagnostic for
-      // preprocessed input as we don't expect it to be used with -std=libc++
-      // anyway.
-      Res.getDiagnosticOpts().Warnings.push_back("no-stdlibcxx-not-found");
-    }
   }
 
   if (LangOpts.CUDA) {