gcc/testsuite/g++.dg/cpp0x/forw_enum8.C

// { dg-do compile { target c++11 } }

//This instatiation is ok
template<typename T> struct S1
{
    enum E : int;
    enum E : T;
};
template struct S1<int>; //ok

//This error is diagnosed at instantiation time
template<typename T> struct S2
{
    enum E : int;   // { dg-error "previous definition" }
    enum E : T;     // { dg-error "different underlying type" }
};
template struct S2<short>; // { dg-message "required from here" }

//This error is diagnosed at compilation time
template<typename T> struct S3
{
    enum E : int;   // { dg-error "previous definition" }
    enum E : short; // { dg-error "different underlying type" }
};