[Bug c++/104712] -fkeep-inline-functions causing link errors (debian but not godbolt?)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104712

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The in-class declaration is the definition if it's inline and has an
initializer.