[Dune-devel] Why does MPIHelper::instance return a non-const reference?

[Oliver Sander]

Dear Dune,

I recently noticed that the method MPIHelper::instance returns a reference
to MPIHelper that is non-const.  However, there is nothing I can do with an MPIHelper
that would require non-constness.  Is there a subtle reason I fail to see?

Unfortunately, adding the const is difficult, because it breaks all code
that captures the return value of 'instance' by a non-const reference.
Should we at least advise our users that what 'instance' returns is
practically a const reference?

Thanks,
Oliver

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