Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -487,6 +487,7 @@
 def ObjCMultipleMethodNames : DiagGroup<"objc-multiple-method-names">;
 def ObjCFlexibleArray : DiagGroup<"objc-flexible-array">;
 def ObjCBoxing : DiagGroup<"objc-boxing">;
+def ObjCBoolBitfield : DiagGroup<"objc-bool-bitfield">;
 def OpenCLUnsupportedRGBA: DiagGroup<"opencl-unsupported-rgba">;
 def UnderalignedExceptionObject : DiagGroup<"underaligned-exception-object">;
 def DeprecatedObjCIsaUsage : DiagGroup<"deprecated-objc-isa-usage">;
@@ -929,6 +930,7 @@
     ObjCMissingSuperCalls,
     ObjCDesignatedInit,
     ObjCFlexibleArray,
+    ObjCBoolBitfield,
     OverloadedVirtual,
     PrivateExtern,
     SelTypeCast,
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5892,6 +5892,9 @@
   " %select{struct|interface|union|class|enum}2">;
 def err_objc_variable_sized_type_not_at_end : Error<
   "field %0 with variable sized type %1 is not at the end of class">;
+def warn_objc_bool_bitfield_has_width_one : Warning<"BOOL bit-field %0 with "
+  " width 1 can not store BOOL's YES value. Change the bitfield type to "
+  "'unsigned' to silence this warning.">, InGroup<ObjCBoolBitfield>;
 def note_next_field_declaration : Note<
   "next field declaration is here">;
 def note_next_ivar_declaration : Note<
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -16513,6 +16513,12 @@
           << FieldName << Value.toString(10)
           << (unsigned)TypeWidth;
     }
+
+    // If Objective C's BOOL type is a signed char (or any signed integer), then
+    // 1 bit is not wide enough to store Objective-C's YES(1) value.
+    if (LangOpts.ObjC && Value == 1 && NSAPIObj->isObjCBOOLType(FieldTy) &&
+        FieldTy->isSignedIntegerType())
+      Diag(FieldLoc, diag::warn_objc_bool_bitfield_has_width_one) << FieldName;
   }
 
   return BitWidth;
Index: clang/test/Misc/warning-wall.c
===================================================================
--- clang/test/Misc/warning-wall.c
+++ clang/test/Misc/warning-wall.c
@@ -78,6 +78,7 @@
 CHECK-NEXT:    -Wobjc-missing-super-calls
 CHECK-NEXT:    -Wobjc-designated-initializers
 CHECK-NEXT:    -Wobjc-flexible-array
+CHECK-NEXT:    -Wobjc-bool-bitfield
 CHECK-NEXT:    -Woverloaded-virtual
 CHECK-NEXT:    -Wprivate-extern
 CHECK-NEXT:    -Wcast-of-sel-type
Index: clang/test/Sema/bitfield.c
===================================================================
--- clang/test/Sema/bitfield.c
+++ clang/test/Sema/bitfield.c
@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify -std=c11 -Wno-unused-value
 
+typedef signed char BOOL;
+
 enum e0; // expected-note{{forward declaration of 'enum e0'}}
 
 struct a {
@@ -28,6 +30,9 @@
 
   _Bool : 2;   // expected-error {{width of anonymous bit-field (2 bits) exceeds width of its type (1 bit)}}
   _Bool h : 5; // expected-error {{width of bit-field 'h' (5 bits) exceeds width of its type (1 bit)}}
+
+  // Make sure this doesn't trigger Objective-C's BOOL bitfield warning.
+  BOOL i : 1;
 };
 
 struct b {unsigned x : 2;} x;
Index: clang/test/SemaObjC/class-bool-bitfield.m
===================================================================
--- /dev/null
+++ clang/test/SemaObjC/class-bool-bitfield.m
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 %s -fobjc-runtime=macosx-fragile-10.5 -fsyntax-only -verify=signed
+// RUN: %clang_cc1 %s -DBOOL_IS_BOOL -fobjc-runtime=macosx-fragile-10.5 -fsyntax-only -verify=bool
+
+#ifdef BOOL_IS_BOOL
+// If BOOL is _Bool then bitfields with a width of 1 are fine.
+typedef _Bool BOOL;
+#else
+typedef signed char BOOL;
+#endif
+
+typedef BOOL BOOL_TYPEDEF;
+
+@interface X {
+  // If BOOL is a signed char then a 1-bit field isn't wide enough to store
+  // YES(1).
+  BOOL f : 1;         // signed-warning {{BOOL bit-field 'f' with width 1 can not store BOOL's YES value.}}
+  BOOL_TYPEDEF g : 1; // signed-warning {{BOOL bit-field 'g' with width 1 can not store BOOL's YES value.}}
+  // Only warn if Objective-C's BOOL type or a typedef for it is used.
+  signed char h : 1;
+  // A width of 2 or more can store YES and NO.
+  BOOL i : 2; // bool-error {{exceeds width of its type}}
+  BOOL j : 3; // bool-error {{exceeds width of its type}}
+}
+@end
+
+struct S {
+  BOOL f : 1; // signed-warning {{BOOL bit-field 'f' with width 1 can not store BOOL's YES value.}}
+};