[Bug c++/68290] g++.dg/concepts/auto1.C FAILs

[ebotcazou at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68290

--- Comment #3 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
This issue is host-dependent, it doesn't reproduce with a cross to 64-bit
SPARC.

The problematic types are:

(gdb) p debug_tree(t1)
 <template_type_parm fb4ccae0 auto type_0 type_6 VOID
    align 8 symtab 0 alias set -1 canonical type fb4ccae0
   index 0 level 1 orig_level 1
    chain <type_decl fb4ccb40 auto>>

(gdb) p debug_tree(t2)
 <template_type_parm fb4cd0e0 auto VOID
    align 8 symtab 0 alias set -1 canonical type fb4cd0e0
   index 0 level 1 orig_level 1
    chain <type_decl fb4cd140 auto>>

and they compare equal according to structural_comptypes but have distinct
TYPE_CANONICAL (themselves actually).

I think that the dependence on the host comes from:

inline hashval_t
auto_hash::hash (tree t)
{
  if (tree c = PLACEHOLDER_TYPE_CONSTRAINTS (t))
    /* Matching constrained-type-specifiers denote the same template
       parameter, so hash the constraint.  */
    return hash_placeholder_constraint (c);
  else
    /* But unconstrained autos are all separate, so just hash the pointer.  */
    return iterative_hash_object (t, 0);
}

and that we have a hash collision on the SPARC machine.

The problem seems to come from comp_template_parms_position:

  /* In C++14 we can end up comparing 'auto' to a normal template
     parameter.  Don't confuse them.  */
  if (cxx_dialect >= cxx14 && (is_auto (t1) || is_auto (t2)))
    return TYPE_IDENTIFIER (t1) == TYPE_IDENTIFIER (t2);

IIUC we should compare t1 and t2 directly here if both are 'auto's.