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

[Erich Keane via cfe-commits]

erichkeane 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)?

I'd assume it was a particularly intentional omission.  The semantics (particularly in the abstract machine) of this in a union is particularly difficult to reason about.

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