Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5179,6 +5179,8 @@
   "%select{struct|interface|union|class|enum}1">;
 def err_flexible_array_has_nontrivial_dtor : Error<
   "flexible array member %0 of type %1 with non-trivial destruction">;
+def err_flexible_array_in_objc_class : Error<
+  "%0 may not be used in interface due to flexible array member">;
 def ext_flexible_array_in_struct : Extension<
   "%0 may not be nested in a struct due to flexible array member">,
   InGroup<FlexibleArrayExtensions>;
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -14990,6 +14990,12 @@
         // Ivars can not have abstract class types
         FD->setInvalidDecl();
       }
+      if (isa<ObjCContainerDecl>(EnclosingDecl) && FDTTy->getDecl()->hasFlexibleArrayMember()) {
+        Diag(FD->getLocation(), diag::err_flexible_array_in_objc_class)
+          << FD->getDeclName();
+        FD->setInvalidDecl();
+        EnclosingDecl->setInvalidDecl();
+      }
       if (Record && FDTTy->getDecl()->hasObjectMember())
         Record->setHasObjectMember(true);
       if (Record && FDTTy->getDecl()->hasVolatileMember())
Index: clang/test/SemaObjC/ivar-sem-check-3.m
===================================================================
--- /dev/null
+++ clang/test/SemaObjC/ivar-sem-check-3.m
@@ -0,0 +1,179 @@
+// RUN: %clang_cc1 -emit-llvm-only -fobjc-arc -verify -Wno-objc-root-class %s
+
+// # Flexible array member, not structs with flexible array members.
+// ## Ivars only in interface.
+@interface LastIVar
+{
+  char flexible[];
+}
+@end
+@implementation LastIVar
+@end
+
+@interface NotLastIVar
+{
+  char flexible[];  // expected error
+  int last;
+}
+@end
+@implementation NotLastIVar
+@end
+
+// ## IVars in implementation.
+@interface LastIVarInImpl
+@end
+@implementation LastIVarInImpl
+{
+  char flexible[];  // expected warning like "Field 'flexible' with variable sized type 'char[]' is not visible to subclasses and can conflict with their fields"
+}
+@end
+
+@interface OtherIVarInImpl
+{
+  char flexible[];  // expected error
+}
+@end
+@implementation OtherIVarInImpl
+{
+  int last;
+}
+@end
+
+@interface NotLastIVarInImpl
+@end
+@implementation NotLastIVarInImpl
+{
+  char flexible[];  // expected error
+  int last;
+}
+@end
+
+// ## Synthesized ivars.
+@interface ExplicitlyNamedValid
+{
+  int _elementsCount;
+  char flexible[];
+}
+@property int count;
+@end
+@implementation ExplicitlyNamedValid
+@synthesize count = _elementsCount;
+@end
+
+@interface ImplicitlyNamedValid
+{
+  int _count;
+  char flexible[];
+}
+@property int count;
+@end
+@implementation ExplicitlyNamedValid
+@synthesize count;
+@end
+
+@interface ExplicitlyNamedInvalid
+{
+  char flexible[];  // expected error
+}
+@property int count;
+@end
+@implementation ExplicitlyNamedInvalid
+@synthesize count = _elementsCount;
+@end
+
+@interface ImplicitlyNamedInvalid
+{
+  char flexible[];  // expected error
+}
+@property int count;
+@end
+@implementation ExplicitlyNamedInvalid
+@synthesize count;
+@end
+
+
+// # Variable sized types.
+struct Packet
+{
+  unsigned int size;
+  char data[];
+};
+
+// ## Ivars only in interface.
+@interface StructLastIVar
+{
+  struct Packet flexible;
+}
+@end
+@implementation StructLastIVar
+@end
+
+@interface StructNotLastIVar
+{
+  struct Packet flexible;  // expected error
+  int last;
+}
+@end
+@implementation StructNotLastIVar
+@end
+
+// ## IVars in implementation.
+@interface StructLastIVarInImpl
+@end
+@implementation StructLastIVarInImpl
+{
+  struct Packet flexible;  // expected warning like "Field 'flexible' with variable sized type 'struct Packet' is not visible to subclasses and can conflict with their fields"
+}
+@end
+
+@interface StructOtherIVarInImpl
+{
+  struct Packet flexible;  // expected error
+}
+@end
+@implementation StructOtherIVarInImpl
+{
+  int last;
+}
+@end
+
+@interface StructNotLastIVarInImpl
+@end
+@implementation StructNotLastIVarInImpl
+{
+  struct Packet flexible;  // expected error
+  int last;
+}
+@end
+
+// # Subclassing.
+@interface FlexibleArrayMemberBase
+{
+  char flexible[];
+}
+@end
+@implementation FlexibleArrayMemberBase
+@end
+
+@interface AddedInInterface : FlexibleArrayMemberBase
+{
+  int last;  // expected warning
+}
+@end
+@implementation AddedInInterface
+@end
+
+@interface AddedInImplementation : FlexibleArrayMemberBase
+@end
+@implementation AddedInImplementation
+{
+  int last;  // expected warning
+}
+@end
+
+@interface SynthesizeIVar : FlexibleArrayMemberBase
+@property int count;
+@end
+@implementation AddedInImplementation
+@synthesize count;  // expected warning
+@end