[Bug c/114931] [14/15 regression] ICE in get_alias_set when building tcl with -std=c23

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114931

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
When we build the pointer type there's already

(gdb) p to_type
$18 = <function_type 0x7ffff6b7d930>
(gdb) p to_type->type_common.canonical 
$15 = <function_type 0x7ffff6b7d690>
(gdb) p to_type->type_common.canonical->type_common.canonical 
$17 = <tree 0x0>

the interesting bit is that build_function_type when doing

  else if (any_noncanonical_p)
    TYPE_CANONICAL (t) = build_function_type (TYPE_CANONICAL (value_type),
                                              canon_argtypes);

to build the canonical type gets an already present function type through
type_has_canon that has TYPE_STRUCTURAL_EQUALITY_P set.  So somehow
it is inconsistent with itself here - at the time we build that function
type maybe_canonicalize_argtypes sets any_structural_p so it seems we
set TYPE_CANONICAL "late" on some types?  In particular both the
pointer and struct type have TYPE_STRUCTURAL_EQUALITY_P set.

It might be tempting to do the following which fixes the testcase but then
this simply means we'll never find types with structural equality in the
hash (note we insert those when they still have themselves as canonical type).
In some sense TYPE_STRUCTURAL_EQUALITY_P means we have not properly unified
types so unifying even less shouldn't make it much worse.  It would be still
interesting to see why we build the function type with original "incomplete"
argument types.

diff --git a/gcc/tree.cc b/gcc/tree.cc
index 83f3bf306af..35bb7713e1f 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -6109,6 +6109,10 @@ type_cache_hasher::equal (type_hash *a, type_hash *b)
          || TYPE_MODE (a->type) != TYPE_MODE (b->type)))
     return false;

+  if (TYPE_STRUCTURAL_EQUALITY_P (a->type)
+      != TYPE_STRUCTURAL_EQUALITY_P (b->type))
+    return false;
+
   switch (TREE_CODE (a->type))
     {
     case VOID_TYPE:


debugging patch:

diff --git a/gcc/tree.cc b/gcc/tree.cc
index 83f3bf306af..35bb7713e1f 100644
--- a/gcc/tree.cc
+++ b/gcc/tree.cc
@@ -7520,8 +7524,11 @@ build_function_type (tree value_type, tree arg_types,
   if (any_structural_p)
     SET_TYPE_STRUCTURAL_EQUALITY (t);
   else if (any_noncanonical_p)
+    {
     TYPE_CANONICAL (t) = build_function_type (TYPE_CANONICAL (value_type),
                                              canon_argtypes);
+    gcc_assert (TYPE_CANONICAL (TYPE_CANONICAL (t)) == TYPE_CANONICAL (t));
+    }

   if (!COMPLETE_TYPE_P (t))
     layout_type (t);