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Did you mean the other way around? I think someArray[] is a flexible array member and someArray[0] is a constraint violation.

You are right. OP has it backwards. gcc treats someArray[0] almost the same as someArray[].

Other compilers might have similar treatment for someArray[0].

Just don't try to have both a zero sized array and a flexible array in the same struct. g++ 5 or earlier will compile that, but g++ 6 will not: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69550