diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
index 8703a67..77cf3b3 100644
--- a/clang/lib/Frontend/InitPreprocessor.cpp
+++ b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -330,6 +330,14 @@
       Builder.defineMacro("__cplusplus", "199711L");
   }
 
+  // In C11 these are environment macros. In C++11 they are only defined
+  // as part of <cuchar>. To prevent breakage when mixing C and C++
+  // code, define these macros unconditionally. We can define them
+  // unconditionally, as Clang always uses UTF-16 and UTF-32 for 16-bit
+  // and 32-bit character literals.
+  Builder.defineMacro("__STDC_UTF_16__", "1");
+  Builder.defineMacro("__STDC_UTF_32__", "1");
+
   if (LangOpts.ObjC1)
     Builder.defineMacro("__OBJC__");