[Bug c++/91364] Implement P0388R4: Permit conversions to arrays of unknown bound

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91364

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Note to myself: while messing with this, perhaps implement DR 330 too for good
measure.  Bug 88128.