[Solved] What does this line of code mean

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This is not standard C++ program. Zero size arrays are not allowed in C & C++. You should use -pedantic-errors command line option if you are using g++ & clang++ compiler to strictly confirm to the standard & disable any compiler extensions.

See live demo here. Clang++ says 

source_file.cpp:7:14: error: zero size arrays are an extension [-Werror,-Wzero-length-array]
    int a1 []{};
             ^
1 error generated.

solved What does this line of code mean