Index: include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCommonKinds.td
+++ include/clang/Basic/DiagnosticCommonKinds.td
@@ -84,6 +84,10 @@
 def err_module_build_disabled: Error<
   "module '%0' is needed but has not been provided, and implicit use of module "
   "files is disabled">, DefaultFatal;
+def err_module_unavailable : Error<
+  "module '%0' %select{is incompatible with|requires}1 feature '%2'">;
+def err_module_header_missing : Error<
+  "%select{|umbrella }0header '%1' not found">;
 def err_module_lock_failure : Error<
   "could not acquire lock file for module '%0'">, DefaultFatal;
 def err_module_lock_timeout : Error<
Index: include/clang/Basic/DiagnosticFrontendKinds.td
===================================================================
--- include/clang/Basic/DiagnosticFrontendKinds.td
+++ include/clang/Basic/DiagnosticFrontendKinds.td
@@ -173,10 +173,6 @@
   "no submodule named %0 in module '%1'; did you mean '%2'?">;
 def warn_missing_submodule : Warning<"missing submodule '%0'">,
   InGroup<IncompleteUmbrella>;
-def err_module_unavailable : Error<
-  "module '%0' %select{is incompatible with|requires}1 feature '%2'">;
-def err_module_header_missing : Error<
-  "%select{|umbrella }0header '%1' not found">;
 def err_module_cannot_create_includes : Error<
   "cannot create includes file for module %0: %1">;
 def warn_module_config_macro_undef : Warning<
Index: include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- include/clang/Basic/DiagnosticLexKinds.td
+++ include/clang/Basic/DiagnosticLexKinds.td
@@ -622,6 +622,8 @@
 def warn_auto_module_import : Warning<
   "treating #%select{include|import|include_next|__include_macros}0 as an "
   "import of module '%1'">, InGroup<AutoImport>, DefaultIgnore;
+def note_implicit_top_level_module_import_here : Note<
+  "submodule of top-level module '%0' implicitly imported here">;
 def warn_uncovered_module_header : Warning<
   "umbrella header for module '%0' does not include header '%1'">, 
   InGroup<IncompleteUmbrella>;
Index: lib/Lex/ModuleMap.cpp
===================================================================
--- lib/Lex/ModuleMap.cpp
+++ lib/Lex/ModuleMap.cpp
@@ -345,13 +345,9 @@
   if (Known != Headers.end()) {
     ModuleMap::KnownHeader Result;
     // Iterate over all modules that 'File' is part of to find the best fit.
-    for (KnownHeader &H : Known->second) {
-      // Cannot use a module if it is unavailable.
-      if (!H.getModule()->isAvailable())
-        continue;
+    for (KnownHeader &H : Known->second)
       if (!Result || isBetterKnownHeader(H, Result))
         Result = H;
-    }
     return MakeResult(Result);
   }
 
Index: lib/Lex/PPDirectives.cpp
===================================================================
--- lib/Lex/PPDirectives.cpp
+++ lib/Lex/PPDirectives.cpp
@@ -1651,6 +1651,28 @@
     }
   }
 
+  // If this include corresponds to a module but that module is
+  // unavailable, diagnose the situation and bail out.
+  if (SuggestedModule && !SuggestedModule.getModule()->isAvailable()) {
+    clang::Module::Requirement Requirement;
+    clang::Module::UnresolvedHeaderDirective MissingHeader;
+    Module *M = SuggestedModule.getModule();
+    // Identify the cause.
+    (void)M->isAvailable(getLangOpts(), getTargetInfo(), Requirement,
+                         MissingHeader);
+    if (MissingHeader.FileNameLoc.isValid()) {
+      Diag(MissingHeader.FileNameLoc, diag::err_module_header_missing)
+          << MissingHeader.IsUmbrella << MissingHeader.FileName;
+      Diag(FilenameTok.getLocation(),
+           diag::note_implicit_top_level_module_import_here)
+          << M->getTopLevelModuleName();
+    } else {
+      Diag(MissingHeader.FileNameLoc, diag::err_module_unavailable)
+          << M->getFullModuleName() << Requirement.second << Requirement.first;
+    }
+    return;
+  }
+
   // Should we enter the source file? Set to false if either the source file is
   // known to have no effect beyond its effect on module visibility -- that is,
   // if it's got an include guard that is already defined or is a modular header
Index: test/Modules/Inputs/declare-use/module.map
===================================================================
--- test/Modules/Inputs/declare-use/module.map
+++ test/Modules/Inputs/declare-use/module.map
@@ -20,14 +20,12 @@
 
 module XE {
   header "e.h"
-  header "unavailable.h"
   use XA
   use XB
 }
 
 module XF {
   header "f.h"
-  header "unavailable.h"
   use XA
   use XB
 }
Index: test/Modules/Inputs/macro-reexport/module.modulemap
===================================================================
--- test/Modules/Inputs/macro-reexport/module.modulemap
+++ test/Modules/Inputs/macro-reexport/module.modulemap
@@ -19,5 +19,4 @@
 }
 module f {
   module f1 { header "f1.h" export * }
-  module f2 { header "f2.h" export * }
 }
Index: test/Modules/missing-header.cpp
===================================================================
--- /dev/null
+++ test/Modules/missing-header.cpp
@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: not %clang_cc1 -x c++ -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/submodules %s 2>&1 | FileCheck %s
+
+// Even if the header we ask for is not missing, if the top-level module
+// containing it has a missing header, then the whole top-level is
+// unavailable and we issue an error.
+
+// CHECK: module.map:15:27: error: header 'missing.h' not found
+// CHECK-DAG: missing-header.cpp:[[@LINE+1]]:10: note: submodule of top-level module 'missing_headers' implicitly imported here
+#include "not_missing.h"
+
+// We should not attempt to build the module.
+// CHECK-NOT: could not build module 'missing_headers'