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

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

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

--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Anyway, such changes are a partial shift towards the model to update derived
types which you said you don't want; it doesn't actually update them, but
basically forces new types after the base type(s) is/are finalized.

Another possibility might be simply in all the spots where we set
TYPE_CANONICAL (t) = something; to add if (TYPE_STRUCTURAL_EQUALITY_P
(TYPE_CANONICAL (t))) SET_TYPE_STRUCTURAL_EQUALITY (t);

On the build_function_type it could be
--- gcc/tree.cc.jj      2024-04-16 09:56:16.463008446 +0200
+++ gcc/tree.cc 2024-05-03 10:21:04.119086667 +0200
@@ -7511,17 +7511,25 @@ build_function_type (tree value_type, tr
   hashval_t hash = type_hash_canon_hash (t);
   t = type_hash_canon (hash, t);

-  /* Set up the canonical type. */
-  any_structural_p   = TYPE_STRUCTURAL_EQUALITY_P (value_type);
-  any_noncanonical_p = TYPE_CANONICAL (value_type) != value_type;
-  canon_argtypes = maybe_canonicalize_argtypes (arg_types,
-                                               &any_structural_p,
-                                               &any_noncanonical_p);
-  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);
+  if (TYPE_CANONICAL (t) == t)
+    {
+      /* Set up the canonical type. */
+      any_structural_p = TYPE_STRUCTURAL_EQUALITY_P (value_type);
+      any_noncanonical_p = TYPE_CANONICAL (value_type) != value_type;
+      canon_argtypes = maybe_canonicalize_argtypes (arg_types,
+                                                   &any_structural_p,
+                                                   &any_noncanonical_p);
+      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);
+         if (TYPE_STRUCTURAL_EQUALITY_P (TYPE_CANONICAL (t)))
+           SET_TYPE_STRUCTURAL_EQUALITY (t);
+       }
+    }

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