Index: include/clang/Basic/DiagnosticGroups.td
===================================================================
--- include/clang/Basic/DiagnosticGroups.td
+++ include/clang/Basic/DiagnosticGroups.td
@@ -207,6 +207,7 @@
                                           [NonModularIncludeInFrameworkModule]>;
 def IncompleteModule : DiagGroup<"incomplete-module",
     [IncompleteUmbrella, NonModularIncludeInModule]>;
+def ModuleMapMissing : DiagGroup<"module-map-missing">;
 
 def InvalidNoreturn : DiagGroup<"invalid-noreturn">;
 def InvalidSourceEncoding : DiagGroup<"invalid-source-encoding">;
Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -611,6 +611,9 @@
 def err_mmap_expected_attribute : Error<"expected an attribute name">;
 def warn_mmap_unknown_attribute : Warning<"unknown attribute '%0'">,
   InGroup<IgnoredAttributes>;
+def warn_module_map_not_found : Warning<
+  "File '%0' specified via -fmodule-map-file not found">,
+  InGroup<ModuleMapMissing>;
 
 def warn_auto_module_import : Warning<
   "treating #%select{include|import|include_next|__include_macros}0 as an "
Index: lib/Lex/HeaderSearch.cpp
===================================================================
--- lib/Lex/HeaderSearch.cpp
+++ lib/Lex/HeaderSearch.cpp
@@ -578,7 +578,9 @@
     for (const auto &Filename : HSOpts->ModuleMapFiles)
       if (const FileEntry *File = FileMgr.getFile(Filename))
         loadModuleMapFile(File, /*IsSystem=*/false);
-    HSOpts->ModuleMapFiles.clear();
+      else
+        Diags.Report(diag::warn_module_map_not_found) << Filename;
+	  HSOpts->ModuleMapFiles.clear();
   }
 
   if (SuggestedModule)
Index: test/Modules/pr21217.cpp
===================================================================
--- test/Modules/pr21217.cpp
+++ test/Modules/pr21217.cpp
@@ -0,0 +1,6 @@
+// RUN: not %clang_cc1 -fmodules -fmodule-map-file=does-not-exist.modulemap -verify %s 2>&1 | \
+// RUN: FileCheck %s
+
+// CHECK: File 'does-not-exist.modulemap' specified via -fmodule-map-file not found
+
+#include "Inputs/private/common.h"