Index: lib/Frontend/InitPreprocessor.cpp
===================================================================
--- lib/Frontend/InitPreprocessor.cpp
+++ lib/Frontend/InitPreprocessor.cpp
@@ -593,9 +593,6 @@
         Builder.defineMacro("OBJC_ZEROCOST_EXCEPTIONS");
     }
 
-    Builder.defineMacro("__OBJC_BOOL_IS_BOOL",
-                        Twine(TI.useSignedCharForObjCBool() ? "0" : "1"));
-
     if (LangOpts.getGC() != LangOptions::NonGC)
       Builder.defineMacro("__OBJC_GC__");
 
@@ -626,6 +623,12 @@
     Builder.defineMacro("IB_DESIGNABLE", "");
   }
 
+  // Define a macro that describes the Objective-C boolean type for both
+  // Objective-C/C++ and plain C code.
+  if (LangOpts.ObjC1 || !LangOpts.CPlusPlus)
+    Builder.defineMacro("__OBJC_BOOL_IS_BOOL",
+                        Twine(TI.useSignedCharForObjCBool() ? "0" : "1"));
+
   if (LangOpts.CPlusPlus)
     InitializeCPlusPlusFeatureTestMacros(LangOpts, Builder);
 
Index: test/Frontend/objc-bool-is-bool.m
===================================================================
--- test/Frontend/objc-bool-is-bool.m
+++ test/Frontend/objc-bool-is-bool.m
@@ -1,6 +1,8 @@
 // RUN: %clang_cc1 -fsyntax-only -E -dM -triple=armv7k-apple-watchos %s | FileCheck --check-prefix=BOOL %s
 // RUN: %clang_cc1 -fsyntax-only -E -dM -triple=x86_64-apple-darwin16 %s | FileCheck --check-prefix=CHAR %s
-// RUN: %clang_cc1 -x c -fsyntax-only -E -dM -triple=x86_64-apple-darwin16 %s | FileCheck --check-prefix=NONE %s
+// RUN: %clang_cc1 -x c -fsyntax-only -E -dM -triple=x86_64-apple-darwin16 %s | FileCheck --check-prefix=CHAR %s
+// RUN: %clang_cc1 -x objective-c++ -fsyntax-only -E -dM -triple=x86_64-apple-darwin16 %s | FileCheck --check-prefix=CHAR %s
+// RUN: %clang_cc1 -x c++ -fsyntax-only -E -dM -triple=x86_64-apple-darwin16 %s | FileCheck --check-prefix=NONE %s
 
 // rdar://21170440