[clang] [Clang] Fix __is_array returning true for zero-sized arrays (PR #86652)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman requested changes to this pull request.

My primary question is: then what is it?

We return true for `__is_aggregrate` (https://godbolt.org/z/67zjeo7Mj), and an aggregate is an array or class type (https://eel.is/c++draft/dcl.init.aggr#1). This isn't a class type... but it is an aggregate... so it must be an array? (We also don't claim it's a pointer or a reference currently... so this thing will be basically invisible to type traits.)

I would think it is an array given that it uses array syntax for declarations and array semantics for accesses, but it's also not an array that's usable so I can see why others say it's not an array. Personally, I think Clang's behavior here makes the most sense -- we claim it's an array, we also claim it's a bounded array, and we claim its extent is zero (https://godbolt.org/z/4GdYTh4GG), and that matches the declaration for the type. So with this change, I'm worried about how type traits can possibly reason about this type -- I'd like to understand better why other implementations decided this isn't an array and what it's classified as with their type traits. Assuming that logic is compelling, there's more work needed here to handle things like claiming it's a bounded array but not an array.

Also, do we need an ABI tag for folks to get the old behavior given that this change almost certainly will impact ABI through type traits?

https://github.com/llvm/llvm-project/pull/86652