[Bug c++/90880] compile error instead of SFINAE with non-public member variables

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90880

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Federico Kircheis from comment #3)
> Sorry if linking to external bug trackers in comments is bad practice, but I
> did not saw any rule about it.

There's no rule against it, it's useful.

> Since it could be interesting for those having this issue, here is the bug
> report for MSVC: 
> 
> https://developercommunity.visualstudio.com/content/problem/607019/incorrect-
> decltype-on-non-public-member-variables.html

Thanks.