Passing an uninitialized object to a function that takes its argument
by const reference is diagnosed by -Wmaybe-uninitialized because most
such functions read the argument.  The exceptions are functions that
don't access the object but instead use its address to compute
a result.  This includes a number of std::array member functions such
as std::array<N>::size() which returns the template argument N.  Such
functions may be candidates for attribute const which also avoids
the warning.  The attribute typically only benefits extern functions
that IPA cannot infer the property from, but in this case it helps
avoid the warning which runs very early on, even without optimization
or inlining.  The attached patch adds the attribute to a subset of
those member functions of std::array.  (It doesn't add it to const
member functions like cbegin() or front() that return a const_iterator
or const reference to the internal data.)

It might be possible to infer this property from inline functions
earlier on than during IPA and avoid having to annotate them explicitly.
That seems like an enhancement worth considering in the future.

Tested on x86_64-linux.

Martin