[clang] [Clang][Sema]: Allow flexible arrays in unions and alone in structs (PR #84428)

[Nick Desaulniers via cfe-commits]

nickdesaulniers wrote:

> C23 6.7.3.2p20: "As a special case, the last member of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. ..."

Thanks! Yeah, I wonder if that could have been "of a structure _or union_ " (as in was that an intentional or unintentional omission)?

https://github.com/llvm/llvm-project/pull/84428