[Bug c++/65719] Link error with constexpr variable template

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65719

--- Comment #4 from Louis Dionne <ldionne.2 at gmail dot com> ---
I added Jason to the CC list since he's the one who committed r213641. Jason,
any clue about how to fix this?