[Sema] Emit an error for using tags with flexible array member as ObjC class ivar.

[vsapsai]

Summary

Incomplete patch intended to make discussion more concrete.

[vsapsai]

• Add test cases for the discussion.

[vsapsai]

• Cover a case when subclass doesn't add ivars.

[vsapsai]

Superseded by actual change https://reviews.llvm.org/D38773

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_flexible_array_virtual_base : Error<
   "flexible array member %0 not allowed in "
   "%select{struct|interface|union|class|enum}1 which has a virtual base class">;
 def err_flexible_array_empty_aggregate : Error<
   "flexible array member %0 not allowed in otherwise empty "
   "%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>;
 def ext_flexible_array_in_array : Extension<
   "%0 may not be used as an array element due to flexible array member">,
   InGroup<FlexibleArrayExtensions>;
 def err_flexible_array_init : Error<
   "initialization of flexible array member is not allowed">;

clang/lib/Sema/SemaDecl.cpp

if (FDTy->isFunctionType()) {
       }
       if (isa<ObjCContainerDecl>(EnclosingDecl) &&
           RequireNonAbstractType(FD->getLocation(), FD->getType(),
                                  diag::err_abstract_type_in_decl,
                                  AbstractIvarType)) {
         // 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())
         Record->setHasVolatileMember(true);
     } else if (FDTy->isObjCObjectType()) {
       /// A field cannot be an Objective-c object
       Diag(FD->getLocation(), diag::err_statically_allocated_object)
         << FixItHint::CreateInsertion(FD->getLocation(), "*");

clang/test/SemaObjC/ivar-sem-check-3.m

+// RUN: %clang_cc1 -emit-llvm-only -fobjc-arc -verify -Wno-objc-root-class %s
+
+// Test using structs with flexible array member as ivars.
+struct Packet
+{
+  unsigned int size;
+  char data[];
+};
+
+@interface INTF_FLEXIBLE_ARRAY_MEMBER
+{
+  // Pointer to a struct with flexible array member is acceptable.
+  struct Packet *packetPtr;
+  struct Packet packet;  // expected-error {{'packet' may not be used in interface due to flexible array member}}
+}
+@end
+// Implementation to trigger ivar layout.
+@implementation INTF_FLEXIBLE_ARRAY_MEMBER
+@end