is taken from the SV's UTF-8ness
p4raw-id: //depot/perl@23800
(including, but not limited to, SVs with a UTF-8 PV formatted with %s,
and characters >255 formatted with %c), the original SV might get
upgraded to UTF-8. Handles 'get' magic, but not 'set' magic. See
-C<sv_catpvf_mg>.
+C<sv_catpvf_mg>. If the original SV was UTF-8, the pattern should be
+valid UTF-8; if the original SV was bytes, the pattern should be too.
void sv_catpvf(SV* sv, const char* pat, ...)
(including, but not limited to, SVs with a UTF-8 PV formatted with %s,
and characters >255 formatted with %c), the original SV might get
upgraded to UTF-8. Handles 'get' magic, but not 'set' magic. See
-C<sv_catpvf_mg>.
+C<sv_catpvf_mg>. If the original SV was UTF-8, the pattern should be
+valid UTF-8; if the original SV was bytes, the pattern should be too.
=cut */