https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68762
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> --- You need to provide a out-of-line copy of foo for example by adding a extern double foo (double d); to foobar.cpp. At least if this were C, not 100% sure about C++. OTOH SIMD support may simply fail to provide proper linkage to the clones.
