c++: Hash placeholder constraint in ctp_hasher

This patch addresses a difference between the hash function and the equality
function for canonical types of template parameters (ctp_hasher). The equality
function uses comptypes (typeck.cc) (with COMPARE_STRUCTURAL) and checks
constraint equality for two auto nodes (typeck.cc:1586), while the hash
function ignores it (pt.cc:4528). This leads to hash collisions that can be
avoided by using `hash_placeholder_constraint` (constraint.cc:1150).

Note that due to the proper handling of hash collisions (hash-table.h:1059),
there is no test case that can distinguish the current implementation from the
proposed one.

	* constraint.cc (hash_placeholder_constraint): Rename to
	iterative_hash_placeholder_constraint.
	(iterative_hash_placeholder_constraint): Rename from
	hash_placeholder_constraint and add the initial val argument.
	* cp-tree.h (hash_placeholder_constraint): Rename to
	iterative_hash_placeholder_constraint.
	(iterative_hash_placeholder_constraint): Renamed from
	hash_placeholder_constraint and add the initial val argument.
	* pt.cc (struct ctp_hasher): Updated to use
	iterative_hash_placeholder_constraint in the case of a valid placeholder
	constraint.
	(auto_hash::hash): Reflect the renaming of hash_placeholder_constraint to
	iterative_hash_placeholder_constraint.

1 files changed, 7 insertions, 2 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 057797f..108e929b 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -4500,7 +4500,12 @@ struct ctp_hasher : ggc_ptr_hash<tree_node>
     val = iterative_hash_object (TEMPLATE_TYPE_LEVEL (t), val);
     val = iterative_hash_object (TEMPLATE_TYPE_IDX (t), val);
     if (TREE_CODE (t) == TEMPLATE_TYPE_PARM)
-      val = iterative_hash_template_arg (CLASS_PLACEHOLDER_TEMPLATE (t), val);
+      {
+	val
+	  = iterative_hash_template_arg (CLASS_PLACEHOLDER_TEMPLATE (t), val);
+	if (tree c = NON_ERROR (PLACEHOLDER_TYPE_CONSTRAINTS (t)))
+	  val = iterative_hash_placeholder_constraint (c, val);
+      }
     if (TREE_CODE (t) == BOUND_TEMPLATE_TEMPLATE_PARM)
       val = iterative_hash_template_arg (TYPE_TI_ARGS (t), val);
     --comparing_specializations;
@@ -29581,7 +29586,7 @@ auto_hash::hash (tree t)
   if (tree c = NON_ERROR (PLACEHOLDER_TYPE_CONSTRAINTS (t)))
     /* Matching constrained-type-specifiers denote the same template
        parameter, so hash the constraint.  */
-    return hash_placeholder_constraint (c);
+    return iterative_hash_placeholder_constraint (c, 0);
   else
     /* But unconstrained autos are all separate, so just hash the pointer.  */
     return iterative_hash_object (t, 0);