[Bug bootstrap/101374] [12 Regression] bootstrap failure varpool.c:490:19: error: array subscript 'varpool_node[0]' is partly outside array bounds of 'varpool_node [0]' [-Werror=array-bounds]

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101374

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
The following fails with -O2 -Wall -Werror after but not before the rev. - not
sure if the reduction is otherwise sensible.

template <typename T> struct reinterpret_is_a_helper {
  template <typename U> static T cast(U p) { return T(p); }
};
template <typename, typename U> bool is_a(U);
template <typename T, typename U> T dyn_cast(U p) {
  if (is_a<T>(p))
    return reinterpret_is_a_helper<T>::cast(p);
  return 0;
}
enum tree_node_structure_enum { TS_DECL_COMMON };
void contains_struct_check(int, tree_node_structure_enum, const char *, int,
                           const char *);
enum availability { AVAIL_NOT_AVAILABLE, AVAIL_AVAILABLE };
struct symtab_node {
  symtab_node *ultimate_alias_target();
  unsigned definition : 1;
  unsigned in_other_partition : 1;
};
struct varpool_node : symtab_node {
  availability get_availability();
  varpool_node *ultimate_alias_target(symtab_node *);
  unsigned : 1;
};
inline symtab_node *symtab_node::ultimate_alias_target() {
  dyn_cast<varpool_node *>(this)->get_availability();
  dyn_cast<varpool_node *>(this)->get_availability();
  return 0;
}
inline varpool_node *varpool_node::ultimate_alias_target(symtab_node *) {
  symtab_node __trans_tmp_1 = *symtab_node::ultimate_alias_target();
  varpool_node *n = dyn_cast<varpool_node *>(&__trans_tmp_1);
  return n;
}
availability varpool_node::get_availability() {
  symtab_node ref;
  if (definition && in_other_partition)
    return AVAIL_NOT_AVAILABLE;
  ultimate_alias_target(&ref);
  contains_struct_check(0, TS_DECL_COMMON, "", 5, __FUNCTION__);
  return AVAIL_AVAILABLE;
}

> /tmp/obj2/prev-gcc/cc1plus -quiet varpool.ii -O2 -Wall -Werror 
In member function 'availability varpool_node::get_availability()',
    inlined from 'symtab_node* symtab_node::ultimate_alias_target()' at
varpool.ii:25:51,
    inlined from 'varpool_node*
varpool_node::ultimate_alias_target(symtab_node*)' at varpool.ii:30:66,
    inlined from 'availability
varpool_node::_ZN12varpool_node16get_availabilityEv.part.0()' at
varpool.ii:38:24:
varpool.ii:36:18: error: array subscript 'varpool_node[0]' is partly outside
array bounds of 'varpool_node [0]' [-Werror=array-bounds]
   36 |   if (definition && in_other_partition)
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
varpool.ii: In member function 'availability
varpool_node::_ZN12varpool_node16get_availabilityEv.part.0()':
varpool.ii:19:8: note: object 'varpool_node::<anonymous>' of size 4
   19 | struct varpool_node : symtab_node {
      |        ^~~~~~~~~~~~
In member function 'availability varpool_node::get_availability()',
    inlined from 'symtab_node* symtab_node::ultimate_alias_target()' at
varpool.ii:26:51,
    inlined from 'varpool_node*
varpool_node::ultimate_alias_target(symtab_node*)' at varpool.ii:30:66,
    inlined from 'availability
varpool_node::_ZN12varpool_node16get_availabilityEv.part.0()' at
varpool.ii:38:24:
varpool.ii:36:18: error: array subscript 'varpool_node[0]' is partly outside
array bounds of 'varpool_node [0]' [-Werror=array-bounds]
   36 |   if (definition && in_other_partition)
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
varpool.ii: In member function 'availability
varpool_node::_ZN12varpool_node16get_availabilityEv.part.0()':
varpool.ii:19:8: note: object 'varpool_node::<anonymous>' of size 4
   19 | struct varpool_node : symtab_node {
      |        ^~~~~~~~~~~~
cc1plus: all warnings being treated as errors