enum_reflect demo repo for enum_traits

This repo contains enumio.hpp, ~100 lines of code to demonstrate usage of enum_traits
library header enumerator_traits.hpp for reflecting enumerated values of an enum type.
It also uses the type_name_pp library to pretty-print type names and enum identifiers,
which in turn depends on the ntbs library for 'constexpr C-string' handling.

In total, there's ~1000 lines of code. C++17. GCC >= 9 or Clang (no MSVC yet).
Here's a link to the v0.1 code on Wandbox online compiler for you to play with.
It takes a couple of seconds to compile (may be antisocial to put on Godbolt).

Given this definition of enum X:

enum class X { Alicia, Boole, Bool=Boole, Stott, Doe=0xFF,
               Min16 = INT16_MIN, Max16 = INT16_MAX,
               Min32 = INT32_MIN, Max32 = INT32_MAX };

then this program:

#include "enumio.hpp"
constexpr auto decl_X = enumio::enum_declaration<X>;

#include <cstdio>
int main() { std::puts( decl_X ); }

prints this 'code-gen' for the enum definition:

enum class X {Min32=-2147483647-1,Min16=-32768,Alicia=0,Boole=1,
Stott=2,Doe=255,Max16=32767,Max32=2147483647}

Sorted!
Here's the code for the enum_declaration variable template:

template <typename E, bool DecOrHex = true, char... s>
inline constexpr auto enum_declaration = []
{
    using U = std::underlying_type_t<E>;
    constexpr format f = DecOrHex ? id_eq_dec : id_eq_hex;

    constexpr auto ul_type_name = ltl::type_name_pp<U>;
    constexpr bool fixed = ltl::is_fixed_enum_v<E>
                            && ! std::is_same_v<U,int>;
    constexpr auto uqname = ltl::type_name_pu<E>;

    constexpr auto _class = cat_if<ltl::is_scoped_enum_v<E>>(" class");
    constexpr auto _name  = cat_if<sizeof(uqname)!=0>(cat(' ',uqname));
    constexpr auto _fixed = cat_if<fixed>(cat(" : ", ul_type_name));
    constexpr auto enum_ids = enumerators<E,f,',',s...>;

    return cat("enum",_class,_name,_fixed," {",enum_ids,'}');
}();

The optional template Args allow to print hexadecimal values or add spacing.

A few points to note:

• Completely constexpr; decl_X is compile-time generated.
• For enumerators with duplicate value, like Boole and Bool,
  only the first-declared id is reflected (on GCC & Clang -
  MSVC currently fails to reflect duplicate-value enumerators).
• INT32_MIN is output as -INT32_MAX-1 (correct code-gen).
• It looks like the full range of a 32-bit underlying value is checked.
  Can this be true?
  No; read the docs.

---

Build

Meson build script provided, e.g. use with ninja backend
(Meson will clone the three dependency repos, assuming network access).

meson build
ninja -C build

Linux Travis gcc-9, clang-7-libc++, -std=c++17