[Bug c++/81211] Unhelpful error messages using template instance with non-copyable type argument

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81211

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Stephen Kell from comment #0)
> (It may also be a bug in libstdc++ that std::function should require this of
> its argument type, although that's not completely bonkers.)

It's explicitly required by the standard.