[ag] fmt::formatter-s factory

[GuillaumeDua]

• Part of [ag] better format, print + as opt-in #134 , [ag] fmt::formatter view composition #262

In a nutshell

• transform
• flatten/join
• depth_enumerated

Motivation

Flatten tuple-like aggregates to ease implementation.

struct A{ int i = 42; char c = 'a'; };
struct B{ float f = .123f; A a; };
struct C{ bool b_ = true; B b; };

// flatten C: bool, float, int, char

Details

Add transformations such as typename ... modifier_ts projection-like.

auto join_view_v = join<indexed, typenamed>(C{});

using result_t = std::tuple<
   fmt::formatter<depth_enumerated_t<0, bool>>,
   fmt::formatter<depth_enumerated_t<1, float>>,
   fmt::formatter<depth_enumerated_t<2, int>>,
   fmt::formatter<depth_enumerated_t<2, char>>,
>;
// or
using result_t = std::tuple<
   fmt::formatter<depth_enumerated_t<0, C>>,
   fmt::formatter<depth_enumerated_t<1, bool>>,
   fmt::formatter<depth_enumerated_t<1, B>>,
   fmt::formatter<depth_enumerated_t<2, float>>,
   fmt::formatter<depth_enumerated_t<2, A>>,
   fmt::formatter<depth_enumerated_t<3, int>>,
   fmt::formatter<depth_enumerated_t<3, char>>,
>;

Design

• WIP (poor's man implementation, use csl::mp::tuple_cat instead): https://godbolt.org/z/T5rxzn35e