PSA: Hidden friends are not reflectable in C++26

Just a curiosity I've come across today, but hidden friends don't seem to be reflectable.

 

Hidden friends are obviously not members of their parent structs, so `meta::members_of` skips them.

Hidden friends also can't be named directly, so `^^hidden_friend` fails with

> error: 'hidden_friend' has not been declared

This seems to match the wording of the standard and isn't just a quirk of the implementation.

 

This also means that /u/hanickadot's [`hana:adl<"hidden_friend">(class_type{})`](https://old.reddit.com/r/cpp/comments/1oaug0f/overload_sets_with_c26s_reflection/) fails to resolve with

> 'res(args...)' would be invalid: type 'hana::overloads<>' does not provide a call operator

In other words, I have good news and bad news.

- Good news: We still can't recreate the core language in the library.
- Bad news: We still can't recreate the core language in the library.

 

EDIT: godbolt links:

- https://godbolt.org/z/jrPdsdYvP - `members_of` demo and "has not been declared" (comment out `getX` implementation to see the output of `members_of`).
- https://compiler-explorer.com/z/eE9GTxzWe - `hana::adl` failing with a hidden friend

https://redd.it/1p7fuu8
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