Bug#906351: dune-istl: FTBFS in buster/sid (error: static assertion failed)

[Ansgar Burchardt]

Control: tag -1 fixed-upstream confirmed
Control: forwarded -1 https://gitlab.dune-project.org/core/dune-istl/merge_requests/216

Santiago Vila writes:
> /usr/include/c++/8/bits/stl_tree.h:457:21: error: static assertion failed: comparison object must be invocable as const
>        static_assert(is_invocable_v<const _Compare&, const _Key&, const _Key&>,
>                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

GCC 8 is stricter than earlier versions here.  It should already be
fixed upstream
(https://gitlab.dune-project.org/core/dune-istl/merge_requests/216).

I'll look at backporting the changes to the Debian package in the next
days.

Ansgar