Add defaulted copy and move operations to format_error and system_error

[denizevrenci]

Another approach to not trigger -Wdeprecated and -Wweak-vtables after the discussion in #1334.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[denizevrenci]

Looks like there are other instances of weak vtables in the test code. It could be fixed with some boilerplate but I am not sure if it is worth it.

[vitaut]

Thanks for the PR. LGTM but there are some CI failures.

[denizevrenci-quoine]

Yes. They are caused by -Wweak-vtables firing in testing code. I will fix them or find a workaround.

[denizevrenci]

I noticed it is possible to suppress the warnings and reduce the sizes of the object files by adding an undefined private virtual member function as follows:

private:
  virtual void avoid_weak_vtable();

This can be used in both test code and the two exceptions that I added the defaulted special members to. It avoids the boilerplate associated with adding defaulted members for rule of 5.

[vitaut]

I noticed it is possible to suppress the warnings and reduce the sizes of the object files by adding an undefined private virtual member function as follows

Great find! I think it's a nicer solution than adding defaulted members. Do you know why it results i smaller object files?

[vitaut]

Thinking more of it, not emitting format_error's vtable may be problematic.

[denizevrenci]

Great find! I think it's a nicer solution than adding defaulted members. Do you know why it results in smaller object files?

I noticed later that it needs the definition of the virtual function even though it is never used.

So, this "solution" with the definitions included created core-test.cc.o 884 bytes bigger and format-test.cc.o 1336 bytes bigger than that of master using clang 9 in MacOS.

I would say it is negligiable considering the total size of 4.8M core-test.cc.o and 15M for format-test.cc.o.

[vitaut]

The test sizes don't matter but the library size does. There were requests to make it smaller and we'll do it at some point. Also adding an unused function is a bit of a hack. I suggest going to the previous version of the PR with defaulted members.

[vitaut]

Thanks!