https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64020
--- Comment #1 from dmalcolm at gcc dot gnu.org ---
builtin_info.decl is NULL for BUILT_IN_CEXPI.
4586 static inline bool
4587 builtin_decl_implicit_p (enum built_in_function fncode)
4588 {
4589 size_t uns_fncode = (size_t)fncode;
4590
4591 gcc_checking_assert (BUILTIN_VALID_P (fncode));
4592 return (builtin_info.decl[uns_fncode] != NULL_TREE
4593 && builtin_info.implicit_p[uns_fncode]);
4594 }
4595
Looks like I may need to do something like what the other frontends do to
register builtins (see. e.g. c-family/c-common.c:c_define_builtins)
