https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65469
--- Comment #2 from janus at gcc dot gnu.org ---
This should be enough to fix it:
Index: gcc/fortran/interface.c
===================================================================
--- gcc/fortran/interface.c (Revision 221530)
+++ gcc/fortran/interface.c (Arbeitskopie)
@@ -1569,7 +1569,7 @@ check_interface0 (gfc_interface *p, const char *in
functions or subroutines. */
if (((!p->sym->attr.function && !p->sym->attr.subroutine)
|| !p->sym->attr.if_source)
- && p->sym->attr.flavor != FL_DERIVED)
+ && p->sym->attr.flavor != FL_DERIVED && *p->sym->name)
{
if (p->sym->attr.external)
gfc_error ("Procedure %qs in %s at %L has no explicit interface",
