Index: clang/lib/Lex/ModuleMap.cpp
===================================================================
--- clang/lib/Lex/ModuleMap.cpp
+++ clang/lib/Lex/ModuleMap.cpp
@@ -379,6 +379,7 @@
 bool ModuleMap::isBuiltinHeader(StringRef FileName) {
   return llvm::StringSwitch<bool>(FileName)
            .Case("float.h", true)
+           .Case("inttypes.h", true)
            .Case("iso646.h", true)
            .Case("limits.h", true)
            .Case("stdalign.h", true)
Index: clang/test/Modules/Inputs/System/usr/include/module.map
===================================================================
--- clang/test/Modules/Inputs/System/usr/include/module.map
+++ clang/test/Modules/Inputs/System/usr/include/module.map
@@ -4,6 +4,11 @@
     header "float.h"
   }
 
+  // In both directories (compiler support version wins, forwards)
+  module inttypes {
+    header "inttypes.h"
+  }
+
   // Only in system headers directory
   module stdio {
     header "stdio.h"
Index: clang/test/Modules/cstd.m
===================================================================
--- clang/test/Modules/cstd.m
+++ clang/test/Modules/cstd.m
@@ -28,3 +28,11 @@
 #  error "bool was not defined!"
 #endif
 
+// Supplied by compiler, which forwards to the "/usr/include" version.
+@import cstd.inttypes;
+#ifndef __CLANG_INTTYPES_H
+#error "__CLANG_INTTYPES_H was not defined!"
+#endif
+#ifndef MY_PRIi32
+#error "MY_PRIi32 was not defined!"
+#endif