I came up with this:

 template <class T> struct var {
     using  hash_type = unsigned;
     using value_type = T;
     
     value_type value;
     const hash_type cached_id;

     constexpr var(const value_type the_value) noexcept 
      : value(the_value)
      , cached_id([]{ static hash_type k; return k++; }())
     {}

     constexpr operator auto() noexcept { return value; }
 };

 template <class T> var(T const&) -> var<T>;

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