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

[federico.kircheis at gmail dot com]

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

--- Comment #1 from Federico Kircheis <federico.kircheis at gmail dot com> ---
After researching a little bit more, I've convinced myself that
`status<c1>::value` should be false, as `decltype` has no special rules for
accessing private data, thus clang is correct.

If someone could confirm this, I could try to make a bug report to the msvc
team