C++ template meta programming - https://www.fluentcpp.com/2017/06/02/write-template-metaprogramming-expressively/

is_assignable.hpp

template<typename T, typename U>
using assign_expression = decltype(std::declval<T&>() = std::declval<U&>());

template<typename T, typename U>
constexpr bool is_assignable = is_detected<assign_expression, T, U>;

is_detected.hpp

template<typename...>
using try_to_instantiate = void;
 
using disregard_this = void;
 
template<template<typename...> class Expression, typename Attempt, typename... Ts>
struct is_detected_impl : std::false_type{};
 
template<template<typename...> class Expression, typename... Ts>
struct is_detected_impl<Expression, try_to_instantiate<Expression<Ts...>>, Ts...> : std::true_type{};
 
template<template<typename...> class Expression, typename... Ts>
constexpr bool is_detected = is_detected_impl<Expression, disregard_this, Ts...>::value;