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.
