Fix spurious warnings about unused private field

[ogoffart]

Summary

If the address of a field is taken as a pointer to member, we should
not warn that the field is not used.

Normaly, yse of fields are done from MemberExpr, but in case of pointer to
member, it is in a DeclRefExpr

[djasper]

Looks good.

cfe/trunk/lib/Sema/SemaExpr.cpp

Sema::BuildDeclRefExpr(ValueDecl *D, QualType Ty, ExprValueKind VK,
 
   MarkDeclRefReferenced(E);
 
   if (getLangOpts().ObjCWeak && isa<VarDecl>(D) &&
       Ty.getObjCLifetime() == Qualifiers::OCL_Weak &&
       !Diags.isIgnored(diag::warn_arc_repeated_use_of_weak, E->getLocStart()))
       recordUseOfEvaluatedWeak(E);
 
+  if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
+    UnusedPrivateFields.remove(FD);
     // Just in case we're building an illegal pointer-to-member.
-  FieldDecl *FD = dyn_cast<FieldDecl>(D);
-  if (FD && FD->isBitField())
+    if (FD->isBitField())
       E->setObjectKind(OK_BitField);
+  }
 
   return E;
 }
 
 /// Decomposes the given name into a DeclarationNameInfo, its location, and
 /// possibly a list of template arguments.
 ///
 /// If this produces template arguments, it is permitted to call

cfe/trunk/test/SemaCXX/warn-unused-private-field.cpp

 
 class EverythingUsed {
  public:
   EverythingUsed() : as_array_index_(0), var_(by_initializer_) {
     var_ = sizeof(sizeof_);
     int *use = &by_reference_;
     int test[2];
     test[as_array_index_] = 42;
+    int EverythingUsed::*ptr = &EverythingUsed::by_pointer_to_member_;
   }
 
   template<class T>
   void useStuff(T t) {
     by_template_function_ = 42;
   }
 
  private:
   int var_;
   int sizeof_;
   int by_reference_;
   int by_template_function_;
   int as_array_index_;
   int by_initializer_;
+  int by_pointer_to_member_;
 };
 
 class HasFeatureTest {
 #if __has_feature(attribute_unused_on_fields)
   int unused_; // expected-warning{{private field 'unused_' is not used}}
   int unused2_ __attribute__((unused)); // no-warning
 #endif
 };