[Bug libstdc++/82716] struct/class vs. tuple_element/tuple_size

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82716

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Marc Mutz from comment #0)
> Here's a stripped-down example: https://godbolt.org/g/sUZxAQ
> Works with -stdlib=libc++: https://godbolt.org/g/bW4u1u

But not if you use 'struct' instead of 'class':
https://godbolt.org/z/kCb32t

The only difference is which class-key causes a warning and which one doesn't.