Index: cfe/trunk/include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticCommonKinds.td
+++ cfe/trunk/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: cfe/trunk/include/clang/Basic/DiagnosticFrontendKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticFrontendKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticFrontendKinds.td
@@ -183,10 +183,6 @@
   "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: cfe/trunk/include/clang/Basic/DiagnosticLexKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticLexKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticLexKinds.td
@@ -623,6 +623,8 @@
 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: cfe/trunk/lib/Lex/ModuleMap.cpp
===================================================================
--- cfe/trunk/lib/Lex/ModuleMap.cpp
+++ cfe/trunk/lib/Lex/ModuleMap.cpp
@@ -320,6 +320,10 @@
 
 static bool isBetterKnownHeader(const ModuleMap::KnownHeader &New,
                                 const ModuleMap::KnownHeader &Old) {
+  // Prefer available modules.
+  if (New.getModule()->isAvailable() && !Old.getModule()->isAvailable())
+    return true;
+
   // Prefer a public header over a private header.
   if ((New.getRole() & ModuleMap::PrivateHeader) !=
       (Old.getRole() & ModuleMap::PrivateHeader))
@@ -349,9 +353,6 @@
       // Prefer a header from the current module over all others.
       if (H.getModule()->getTopLevelModule() == CompilingModule)
         return MakeResult(H);
-      // Cannot use a module if it is unavailable.
-      if (!H.getModule()->isAvailable())
-        continue;
       if (!Result || isBetterKnownHeader(H, Result))
         Result = H;
     }
Index: cfe/trunk/lib/Lex/PPDirectives.cpp
===================================================================
--- cfe/trunk/lib/Lex/PPDirectives.cpp
+++ cfe/trunk/lib/Lex/PPDirectives.cpp
@@ -1674,6 +1674,29 @@
           getLangOpts().CurrentModule &&
       SuggestedModule.getModule()->getTopLevelModuleName() !=
           getLangOpts().ImplementationOfModule) {
+
+    // If this include corresponds to a module but that module is
+    // unavailable, diagnose the situation and bail out.
+    if (!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;
+      } else {
+        Diag(M->DefinitionLoc, diag::err_module_unavailable)
+            << M->getFullModuleName() << Requirement.second << Requirement.first;
+      }
+      Diag(FilenameTok.getLocation(),
+           diag::note_implicit_top_level_module_import_here)
+          << M->getTopLevelModuleName();
+      return;
+    }
+
     // Compute the module access path corresponding to this module.
     // FIXME: Should we have a second loadModule() overload to avoid this
     // extra lookup step?
Index: cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_header/not_missing.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_header/not_missing.h
+++ cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_header/not_missing.h
@@ -0,0 +1 @@
+// missing_header/not_missing.h
Index: cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_requirement.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_requirement.h
+++ cfe/trunk/test/Modules/Inputs/auto-import-unavailable/missing_requirement.h
@@ -0,0 +1 @@
+// missing_requirement.h
Index: cfe/trunk/test/Modules/Inputs/auto-import-unavailable/module.modulemap
===================================================================
--- cfe/trunk/test/Modules/Inputs/auto-import-unavailable/module.modulemap
+++ cfe/trunk/test/Modules/Inputs/auto-import-unavailable/module.modulemap
@@ -0,0 +1,19 @@
+module missing_header {
+  module missing { header "missing_header/missing.h" }
+  module error_importing_this { header "missing_header/not_missing.h" }
+}
+
+module nonrequired_missing_header {
+  module unsatisfied_requires {
+    requires nonexistent_feature
+    header "nonrequired_missing_header/missing.h"
+  }
+  module fine_to_import {
+    header "nonrequired_missing_header/not_missing.h"
+  }
+}
+
+module missing_requirement {
+  requires nonexistent_feature
+  header "missing_requirement.h"
+}
Index: cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/not_missing.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/not_missing.h
+++ cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/not_missing.h
@@ -0,0 +1 @@
+// nonrequired_missing_header/not_missing.h
Index: cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/requires_feature_you_dont_have.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/requires_feature_you_dont_have.h
+++ cfe/trunk/test/Modules/Inputs/auto-import-unavailable/nonrequired_missing_header/requires_feature_you_dont_have.h
@@ -0,0 +1 @@
+// nonrequired_missing_header/requires_feature_you_dont_have.h
Index: cfe/trunk/test/Modules/Inputs/available-is-better/available-is-better.h
===================================================================
--- cfe/trunk/test/Modules/Inputs/available-is-better/available-is-better.h
+++ cfe/trunk/test/Modules/Inputs/available-is-better/available-is-better.h
@@ -0,0 +1,2 @@
+#pragma once
+int available;
Index: cfe/trunk/test/Modules/Inputs/available-is-better/module.modulemap
===================================================================
--- cfe/trunk/test/Modules/Inputs/available-is-better/module.modulemap
+++ cfe/trunk/test/Modules/Inputs/available-is-better/module.modulemap
@@ -0,0 +1,17 @@
+// There is some order-dependence to how clang chooses modules, so make
+// sure that both the first and last modules here are ones that would
+// cause a test failure if they were picked.
+
+module unavailable_before {
+  requires nonexistent_feature
+  header "available-is-better.h"
+}
+
+module available {
+  header "available-is-better.h"
+}
+
+module unavailable_after {
+  requires nonexistent_feature
+  header "available-is-better.h"
+}
Index: cfe/trunk/test/Modules/Inputs/declare-use/module.map
===================================================================
--- cfe/trunk/test/Modules/Inputs/declare-use/module.map
+++ cfe/trunk/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: cfe/trunk/test/Modules/Inputs/macro-reexport/module.modulemap
===================================================================
--- cfe/trunk/test/Modules/Inputs/macro-reexport/module.modulemap
+++ cfe/trunk/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: cfe/trunk/test/Modules/auto-import-unavailable.cpp
===================================================================
--- cfe/trunk/test/Modules/auto-import-unavailable.cpp
+++ cfe/trunk/test/Modules/auto-import-unavailable.cpp
@@ -0,0 +1,47 @@
+// RUN: rm -rf %t
+// RUN: not %clang_cc1 -x c++ -Rmodule-build -DMISSING_HEADER -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/auto-import-unavailable %s 2>&1 | FileCheck %s --check-prefix=MISSING-HEADER
+// RUN: %clang_cc1 -x c++ -Rmodule-build -DNONREQUIRED_MISSING_HEADER -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/auto-import-unavailable %s 2>&1 | FileCheck %s --check-prefix=NONREQUIRED-MISSING-HEADER
+// RUN: not %clang_cc1 -x c++ -Rmodule-build -DMISSING_REQUIREMENT -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/auto-import-unavailable %s 2>&1 | FileCheck %s --check-prefix=MISSING-REQUIREMENT
+
+#ifdef MISSING_HEADER
+
+// 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.
+
+// MISSING-HEADER: module.modulemap:2:27: error: header 'missing_header/missing.h' not found
+// MISSING-HEADER-DAG: auto-import-unavailable.cpp:[[@LINE+1]]:10: note: submodule of top-level module 'missing_header' implicitly imported here
+#include "missing_header/not_missing.h"
+
+// We should not attempt to build the module.
+// MISSING-HEADER-NOT: remark: building module
+
+#endif // #ifdef MISSING_HEADER
+
+
+#ifdef NONREQUIRED_MISSING_HEADER
+
+// However, if the missing header is dominated by an unsatisfied
+// `requires`, then that is acceptable.
+// This also tests that an unsatisfied `requires` elsewhere in the
+// top-level module doesn't affect an available module.
+
+// NONREQUIRED-MISSING-HEADER: auto-import-unavailable.cpp:[[@LINE+2]]:10: remark: building module 'nonrequired_missing_header'
+// NONREQUIRED-MISSING-HEADER: auto-import-unavailable.cpp:[[@LINE+1]]:10: remark: finished building module 'nonrequired_missing_header'
+#include "nonrequired_missing_header/not_missing.h"
+
+#endif // #ifdef NONREQUIRED_MISSING_HEADER
+
+
+#ifdef MISSING_REQUIREMENT
+
+// If the header is unavailable due to a missing requirement, an error
+// should be emitted if a user tries to include it.
+
+// MISSING-REQUIREMENT:module.modulemap:16:8: error: module 'missing_requirement' requires feature 'nonexistent_feature'
+// MISSING-REQUIREMENT: auto-import-unavailable.cpp:[[@LINE+1]]:10: note: submodule of top-level module 'missing_requirement' implicitly imported here
+#include "missing_requirement.h"
+
+// MISSING-REQUIREMENT-NOT: remark: building module
+
+#endif // #ifdef MISSING_REQUIREMENT
Index: cfe/trunk/test/Modules/available-is-better.cpp
===================================================================
--- cfe/trunk/test/Modules/available-is-better.cpp
+++ cfe/trunk/test/Modules/available-is-better.cpp
@@ -0,0 +1,5 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -Rmodule-build -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/available-is-better %s 2>&1 | FileCheck %s
+
+#include "available-is-better.h"
+// CHECK: remark: building module