Three constant wrappers in C++26?

If my understanding is correct, we will have 3 compile time value wrappers in C++26:

- std::integral_constant
- std::nontype_t
- std::constant_wrapper

Note: I think there's some discussion in renaming nontype_t to something else, like constant_arg_t or fn_t, nevertheless it'll remain separate from constant_wrapper and integral_constant

I think this mess is worse than that of functions (function, move_only_function, copyable_function). With functions, at least the rule of thumb is "avoid function; use the other two". But with the constant wrappers? It seems that each of them has their legit use case and none is getting deprecated.

Which one should be used at function boundary? Some libraries already made the choice of integral_constant such as boost.PFR. Other libraries may make a different choice. And since these three are not inter-convertible, I'm afraid this situation will create more work than needed for library writers and/or users.

https://redd.it/1o4kbpz
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