env and prop are relying on deprecated behavior

[ericniebler]

In p3325r3, prop is defined with a deleted copy-assignment operator, as follows:

  template<class Query, class Value>
  struct prop {
    Query query;    // exposition only
    Value value;    // exposition only

    static_assert(see below);

    constexpr const Value& query(Query) const noexcept {
      return value;
    }

    prop& operator=(const prop&) = delete;
  };

  template<class Query, class Value>
  prop(Query, Value) -> prop<Query, unwrap_reference_t<Value>>;

This causes clang to issue the diagnostic:

definition of implicit copy constructor for 'prop<>' is deprecated because
it has a user-declared copy assignment operator

We can't "fix" the code by defining or defaulting a copy constructor; that would break aggregate initialization. I don't know what else to do in my own code except suppress the warning and hope the behavior is never removed.