[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 #3 from Louis Dionne <ldionne.2 at gmail dot com> ---
This bug is a real, real pain. I'm willing to help fixing it and/or to provide
test cases, but it'll take me forever if someone on your side does not assist,
since I don't know the GCC code base. I've looked at the changes introduced in
r213641 and I wasn't able to see an obvious cause for the bug, but then again I
don't know GCC.

What I can tell is this: If you look at the produced assembly
(https://goo.gl/Uxc8Dl), you can clearly see that no symbol is ever emitted for
`f<int>`. When you explicitly instantiate `f<int>`, however, the symbol is
emitted. If `f<int>` is not constexpr, the symbol is also emitted.