Index: include/clang/Lex/HeaderSearch.h
===================================================================
--- include/clang/Lex/HeaderSearch.h
+++ include/clang/Lex/HeaderSearch.h
@@ -95,6 +95,12 @@
   /// external storage.
   const IdentifierInfo *ControllingMacro;
 
+  /// List of modules that import from this header. Since the number of modules
+  /// using the same FileEntry is usually small (2 or 3 at most, often related
+  /// to builtins) this should be enough. Choose a more appropriate data
+  /// structure in case the requirement changes.
+  llvm::SmallVector<const Module *, 2> ModulesUsingHdr;
+
   /// \brief If this header came from a framework include, this is the name
   /// of the framework.
   StringRef Framework;
@@ -110,6 +116,15 @@
   const IdentifierInfo *
   getControllingMacro(ExternalPreprocessorSource *External);
 
+  /// \brief Add a module \p M that uses this header.
+  void addModulesUsingHdr(const Module *M);
+  /// \brief Add modules that uses this header from \p OtherModulesUsingHdr.
+  void addModulesUsingHdr(ArrayRef<const Module *> OtherModulesUsingHdr);
+  /// \return The list of modules importing from this header.
+  ArrayRef<const Module *> getModulesUsingHdr() const;
+  /// \return true if \p M is imports this header.
+  bool isModuleUsingHdr(const Module *M);
+
   /// \brief Determine whether this is a non-default header file info, e.g.,
   /// it corresponds to an actual header we've included or tried to include.
   bool isNonDefault() const {
Index: include/clang/Lex/Preprocessor.h
===================================================================
--- include/clang/Lex/Preprocessor.h
+++ include/clang/Lex/Preprocessor.h
@@ -500,6 +500,7 @@
   typedef llvm::DenseMap<const IdentifierInfo *, MacroState> MacroMap;
 
   friend class ASTReader;
+  friend class HeaderSearch;
 
   struct SubmoduleState;
 
Index: lib/Lex/HeaderSearch.cpp
===================================================================
--- lib/Lex/HeaderSearch.cpp
+++ lib/Lex/HeaderSearch.cpp
@@ -52,6 +52,27 @@
   return ControllingMacro;
 }
 
+void HeaderFileInfo::addModulesUsingHdr(const Module *M) {
+  for (auto *Mod : ModulesUsingHdr)
+    if (Mod == M)
+      return;
+  ModulesUsingHdr.push_back(M);
+}
+void HeaderFileInfo::addModulesUsingHdr(
+    ArrayRef<const Module *> OtherModulesUsingHdr) {
+  for (auto *M : OtherModulesUsingHdr)
+    addModulesUsingHdr(M);
+}
+ArrayRef<const Module *> HeaderFileInfo::getModulesUsingHdr() const {
+  return ModulesUsingHdr;
+}
+bool HeaderFileInfo::isModuleUsingHdr(const Module *M) {
+  for (auto *Mod : ModulesUsingHdr)
+    if (Mod == M)
+      return true;
+  return false;
+}
+
 ExternalHeaderFileInfoSource::~ExternalHeaderFileInfoSource() {}
 
 HeaderSearch::HeaderSearch(IntrusiveRefCntPtr<HeaderSearchOptions> HSOpts,
@@ -969,6 +990,8 @@
   HFI.isPragmaOnce |= OtherHFI.isPragmaOnce;
   HFI.isModuleHeader |= OtherHFI.isModuleHeader;
   HFI.NumIncludes += OtherHFI.NumIncludes;
+  if (OtherHFI.isImport || OtherHFI.isPragmaOnce)
+    HFI.addModulesUsingHdr(OtherHFI.getModulesUsingHdr());
 
   if (!HFI.ControllingMacro && !HFI.ControllingMacroID) {
     HFI.ControllingMacro = OtherHFI.ControllingMacro;
@@ -1089,9 +1112,18 @@
     if (FileInfo.NumIncludes) return false;
   } else {
     // Otherwise, if this is a #include of a file that was previously #import'd
-    // or if this is the second #include of a #pragma once file, ignore it.
-    if (FileInfo.isImport)
-      return false;
+    // or if this is the second #include of a #pragma once file, ignore it:
+    //  - if there not associated module or the module already imports
+    //    from this header.
+    //  - if any module associated with this header is visible.
+    if (FileInfo.isImport) {
+      if (!M || FileInfo.isModuleUsingHdr(M))
+        return false;
+      ArrayRef<const Module *> OtherMods = FileInfo.getModulesUsingHdr();
+      for (auto *OtherMod : OtherMods)
+        if (PP.CurSubmoduleState->VisibleModules.isVisible(OtherMod))
+          return false;
+    }
   }
 
   // Next, check to see if the file is wrapped with #ifndef guards.  If so, and
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -1688,6 +1688,8 @@
     Module::Header H = { key.Filename, FileMgr.getFile(Filename) };
     ModMap.addHeader(Mod, H, HeaderRole, /*Imported*/true);
     HFI.isModuleHeader |= !(HeaderRole & ModuleMap::TextualHeader);
+    if (HFI.isImport)
+      HFI.addModulesUsingHdr(Mod);
   }
 
   // This HeaderFileInfo was externally loaded.
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/cstddef
===================================================================
--- /dev/null
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/cstddef
@@ -0,0 +1,9 @@
+#ifndef _LIBCPP_CSTDDEF
+#define _LIBCPP_CSTDDEF
+
+#include <stddef.h>
+#include <type_traits>
+
+typedef ptrdiff_t my_ptrdiff_t;
+
+#endif
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/math.h
===================================================================
--- test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/math.h
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/math.h
@@ -4,4 +4,6 @@
 #include_next <math.h>
 template<typename T> T abs(T t) { return (t < 0) ? -t : t; }
 
+#include <type_traits>
+
 #endif
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/module.modulemap
===================================================================
--- test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/module.modulemap
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/module.modulemap
@@ -6,5 +6,7 @@
   // FIXME: remove "textual" from stdint module below once the issue
   // between umbrella headers and builtins is resolved.
   module stdint { textual header "stdint.h" export * }
+  module type_traits { header "type_traits" export * }
+  module cstddef { header "cstddef" export * }
   module __config { header "__config" export * }
 }
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/stddef.h
===================================================================
--- test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/stddef.h
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/stddef.h
@@ -2,5 +2,6 @@
 #define LIBCXX_STDDEF_H
 
 #include <__config>
+#include_next <stddef.h>
 
 #endif
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/type_traits
===================================================================
--- /dev/null
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/c++/v1/type_traits
@@ -0,0 +1,6 @@
+#ifndef _LIBCPP_TYPE_TRAITS
+#define _LIBCPP_TYPE_TRAITS
+
+#include <cstddef>
+
+#endif
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/module.modulemap
===================================================================
--- test/Modules/Inputs/libc-libcxx/sysroot/usr/include/module.modulemap
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/module.modulemap
@@ -5,4 +5,12 @@
   module stdint { header "stdint.h" export * }
   module stdio { header "stdio.h" export * }
   module util { header "util.h" export * }
+  module POSIX {
+    module sys {
+      module types {
+        umbrella header "sys/_types/_types.h"
+        export *
+      }
+    }
+  }
 }
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/stddef.h
===================================================================
--- test/Modules/Inputs/libc-libcxx/sysroot/usr/include/stddef.h
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/stddef.h
@@ -1 +1,6 @@
-// stddef.h
+#ifndef __STDDEF_H__
+#define __STDDEF_H__
+
+#include "sys/_types/_ptrdiff_t.h"
+
+#endif
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/sys/_types/_ptrdiff_t.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/sys/_types/_ptrdiff_t.h
@@ -0,0 +1,4 @@
+#ifndef _PTRDIFF_T
+#define _PTRDIFF_T
+typedef int * ptrdiff_t;
+#endif /* _PTRDIFF_T */
Index: test/Modules/Inputs/libc-libcxx/sysroot/usr/include/sys/_types/_types.h
===================================================================
--- /dev/null
+++ test/Modules/Inputs/libc-libcxx/sysroot/usr/include/sys/_types/_types.h
@@ -0,0 +1,6 @@
+#ifndef _SYS_TYPES_UMBRELLA
+#define _SYS_TYPES_UMBRELLA
+
+#include "_ptrdiff_t.h"
+
+#endif
Index: test/Modules/builtin-import.mm
===================================================================
--- /dev/null
+++ test/Modules/builtin-import.mm
@@ -0,0 +1,12 @@
+// REQUIRES: system-darwin
+
+// RUN: rm -rf %t
+// RUN: %clang -cc1 -fsyntax-only -nostdinc++ -isysroot %S/Inputs/libc-libcxx/sysroot -isystem %S/Inputs/libc-libcxx/sysroot/usr/include/c++/v1 -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -x objective-c++ -fmodules-local-submodule-visibility %s
+
+#include <stdio.h>
+#include <stddef.h>
+#include <cstddef>
+
+typedef ptrdiff_t try1_ptrdiff_t;
+typedef my_ptrdiff_t try2_ptrdiff_t;
+