[Bug c++/69672] New: Useless -Wenum-compare when comparing enum vaue against the same value in enum hack

["wipedout at yandex dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69672

            Bug ID: 69672
           Summary: Useless -Wenum-compare when comparing enum vaue
                    against the same value in enum hack
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wipedout at yandex dot ru
  Target Milestone: ---

I'm trying this on http://gcc.godbolt.org/ the version is "x86 gcc 5.3.0", the
options line is "-O3 -Wall -std=c++11" The code is as follows:

#include <exception>

enum Magic {
  One, Two, Three
};

class Base {
public:
  virtual Magic GetType() = 0;
};

class ClassOne : public Base {
public:
  enum {
    MagicValue = One
  };
  Magic GetType() { return One; }
};

class ClassTwo : public Base {
public:
  enum {
    MagicValue = Two
  };
  Magic GetType() { return Two; }
};

template
<class Target>
Target* Cast( Base* from)
{
  if( from->GetType() != Target::MagicValue )
    throw new std::exception();
  return static_cast<Target*>( from );
}

void test()
{
  ClassOne obj;
  Cast<ClassOne>( &obj ); 
}

I get the following:

warning: comparison between 'enum Magic' and 'enum ClassOne::<anonymous>'
[-Wenum-compare]

Well, yes, these two are indeed distinct warnings but code uses so-called enum
hack to define a compile-time constant. The enum arising from the enum hack is
a strict subset of the original enum. There's no room for error here. The
warning is useless in this scenario.