Fix Attribute copy/move constructors

[franzpoeschel]

Copy and move constructors of the Attribute class were accidentally custom-defined by having a constructor of the form:

    template <typename T>
    Attribute(T &&val);

Hence, be a bit more careful:

    template <typename T>
    Attribute(std::enable_if_t<
              // If T is `Attribute` or `Attribute const &`, this constructor
              // should not be used, but instead the move/copy constructors
              !std::is_same_v<
                  std::remove_cv_t<std::remove_reference_t<T>>,
                  Attribute>,
              T> &&val)

[ax3l]

Would specializing be simpler/cleaner than enable if?

[franzpoeschel]

That might be better, yes. We would need to do this for every openPMD datatype then, though.
Probably best at that point to introduce something like the datatype macros that ADIOS2 has? openPMD_FOREACH_DATATYPE or so?

[ax3l]

Is that needed? The specialization could be something like

template <typename T>
Attribute(Attribute<T> const &) = default;

template <typename T>
Attribute(Attribute<T> &&) = default;

[franzpoeschel]

Template specialization doesn't work in this case: With template, we cannot use default, without template, it's not a specialization, but an overload that is never chosen.
Using a specialization doesn't fix the underlying problem. In the current form, the constructor template <typename T> Attribute(T &&val) accepts anything, but is supposed to work only for all openPMD datatypes. That's a wrong interface.
Since the Intel Compiler doesn't understand this use of enable_if, I'd suggest using a macro.

[franzpoeschel]

The best would be a C++20 concept here though :D

[ax3l]

What I propose is no template specialization but an overload for the constructors. Did you try if that works?

[ax3l]

I see you did another approach with macros now ;-)

[franzpoeschel]

What I propose is no template specialization but an overload for the constructors. Did you try if that works?

That's exactly what this PR does. Just, it doesn't write all ~30 constructors explicitly, but uses a macro for that instead.