C++ union element size based on other's element size

Consider the following piece of code:

struct S
{
    int arr1[10];
    char arr2[sizeof(arr1)];
};

It compiles successfully with gcc 4.9.2 in c++03 and c++11 mode. However when I change S to be a template like so:

template <size_t N>
struct S
{
    union
    {   
        int arr1[N];
        char arr2[sizeof(arr1)];
    };  
};

I get the following error output:

error: int S<10ul>::<anonymous union>::arr1 [10]’ is inaccessible

int arr1[N];

error: within this context

char arr2[sizeof(arr1)];

Clang compiles both versions only in c++11 mode. I am curious what is the correct behavior here. Maybe should I explicitly state that arr2 size is sizeof(int) * N?