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template<int a,int b=a>struct t{enum{x=t<a,~-b>::x+!(a%b)};};
template<int b>struct t<b,0>{enum{x};};
int _[t<1>::x==2];
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clang++ -std=c++1z -Wall -pedantic-errors -pthread main.cpp && ./a.out

main.cpp:3:11: error: zero size arrays are an extension [-Werror,-Wzero-length-array]
    int _[t<1>::x==2];
          ^~~~~~~~~~
1 error generated.