[Bug c++/103769] [11/12 Regression] checking ICE in hashtab_chk_error with alias template and pack expansion after r11-7931

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103769

--- Comment #6 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
Trying to understand what the error means:

Crash happens when we look up entry in
  gcc/cp/pt.c:static GTY (()) spec_hash_table *type_specializations;

It's a hash table of entries of spec_entry type:
  struct GTY((for_user)) spec_entry
  {
    tree tmpl;  /* The general template this is a specialization of.  */
    tree args;  /* The args for this (maybe-partial) specialization.  */
    tree spec;  /* The specialization itself.  */
  };

spec_hasher type defines both hasher and equality. hash is defined on fields
'tmpl' and 'args', equality is defined on 'tmpl', 'args' and sometimes 'spec':

bool
spec_hasher::equal (spec_entry *e1, spec_entry *e2)
{
  int equal;

  ++comparing_specializations;
  ++comparing_dependent_aliases;
  ++processing_template_decl;
  equal = (e1->tmpl == e2->tmpl
           && comp_template_args (e1->args, e2->args));
  if (equal && flag_concepts
      /* tmpl could be a FIELD_DECL for a capture pack.  */
      && TREE_CODE (e1->tmpl) == TEMPLATE_DECL
      && VAR_P (DECL_TEMPLATE_RESULT (e1->tmpl))
      && uses_template_parms (e1->args))
    {
      /* Partial specializations of a variable template can be distinguished by
         constraints.  */
      tree c1 = e1->spec ? get_constraints (e1->spec) : NULL_TREE;
      tree c2 = e2->spec ? get_constraints (e2->spec) : NULL_TREE;
      equal = equivalent_constraints (c1, c2);
    }
  --processing_template_decl;
  --comparing_dependent_aliases;
  --comparing_specializations;

  return equal;
}

static hashval_t
hash_tmpl_and_args (tree tmpl, tree args)
{
  hashval_t val = iterative_hash_object (DECL_UID (tmpl), 0);
  return iterative_hash_template_arg (args, val);
}

hashval_t
spec_hasher::hash (spec_entry *e)
{
  return hash_tmpl_and_args (e->tmpl, e->args);
}

How can hash differ when values match?