c++: partial ordering and dep alias tmpl specs [PR90679]

During partial ordering, we want to look through dependent alias
template specializations within template arguments and otherwise
treat them as opaque in other contexts (see e.g. r7-7116-g0c942f3edab108
and r11-7011-g6e0a231a4aa240).  To that end template_args_equal was
given a partial_order flag that controls this behavior.  This flag
does the right thing when a dependent alias template specialization
appears as template argument of the partial specialization, e.g. in

  template<class T, class...> using first_t = T;
  template<class T> struct traits;
  template<class T> struct traits<first_t<T, T&>> { }; // #1
  template<class T> struct traits<first_t<const T, T&>> { }; // #2

we correctly consider #2 to be more specialized than #1.  But if the
alias specialization appears as a nested template argument of another
class template specialization, e.g. in

  template<class T> struct traits<A<first_t<T, T&>>> { }; // #1
  template<class T> struct traits<A<first_t<const T, T&>>> { }; // #2

then we incorrectly consider #1 and #2 to be unordered.  This is because

  1. we don't propagate the flag to recursive template_args_equal calls
  2. we don't use structural equality for class template specializations
     written in terms of dependent alias template specializations

This patch fixes the first issue by turning the partial_order flag into
a global.  This patch fixes the second issue by making us propagate
structural equality appropriately when building a class template
specialization.  In passing this patch also improves hashing of
specializations that use structural equality.

	PR c++/90679

gcc/cp/ChangeLog:

	* cp-tree.h (comp_template_args): Remove partial_order parameter.
	(template_args_equal): Likewise.
	* pt.cc (comparing_for_partial_ordering): New global flag.
	(iterative_hash_template_arg) <case tcc_type>: Hash the template
	and arguments for specializations that use structural equality.
	(template_args_equal): Remove partial order parameter and
	use comparing_for_partial_ordering instead.
	(comp_template_args): Likewise.
	(comp_template_args_porder): Set comparing_for_partial_ordering
	instead.  Make static.
	(any_template_arguments_need_structural_equality_p): Return true
	for an argument that's a dependent alias template specialization
	or a class template specialization that itself needs structural
	equality.
	* tree.cc (cp_tree_equal) <case TREE_VEC>: Adjust call to
	comp_template_args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-75a.C: New test.
	* g++.dg/cpp0x/alias-decl-75b.C: New test.

gcc/cp/cp-tree.h

@@ -7507,8 +7507,8 @@ extern int template_class_depth			(tree);
 extern int is_specialization_of			(tree, tree);
 extern bool is_specialization_of_friend		(tree, tree);
 extern bool comp_template_args			(tree, tree, tree * = NULL,
-						 tree * = NULL, bool = false);
-extern int template_args_equal                  (tree, tree, bool = false);
+						 tree * = NULL);
+extern int template_args_equal                  (tree, tree);
 extern tree maybe_process_partial_specialization (tree);
 extern tree most_specialized_instantiation	(tree);
 extern tree most_specialized_partial_spec       (tree, tsubst_flags_t, bool = false);

gcc/cp/pt.cc

@@ -1646,6 +1646,12 @@ register_specialization (tree spec, tree tmpl, tree args, bool is_friend,
 int comparing_specializations;
 int comparing_dependent_aliases;
 
+/* Whether we are comparing template arguments during partial ordering
+   (and therefore want the comparison to look through dependent alias
+   template specializations).  */
+
+static int comparing_for_partial_ordering;
+
 /* Returns true iff two spec_entry nodes are equivalent.  */
 
 bool
@@ -1894,6 +1900,11 @@ iterative_hash_template_arg (tree arg, hashval_t val)
 	default:
 	  if (tree canonical = TYPE_CANONICAL (arg))
 	    val = iterative_hash_object (TYPE_HASH (canonical), val);
+	  else if (tree ti = TYPE_TEMPLATE_INFO (arg))
+	    {
+	      val = iterative_hash_template_arg (TI_TEMPLATE (ti), val);
+	      val = iterative_hash_template_arg (TI_ARGS (ti), val);
+	    }
 	  break;
 	}
 
@@ -9322,7 +9333,7 @@ class_nttp_const_wrapper_p (tree t)
 /* Returns 1 if template args OT and NT are equivalent.  */
 
 int
-template_args_equal (tree ot, tree nt, bool partial_order /* = false */)
+template_args_equal (tree ot, tree nt)
 {
   if (nt == ot)
     return 1;
@@ -9345,7 +9356,7 @@ template_args_equal (tree ot, tree nt, bool partial_order /* = false */)
      During partial ordering, however, we need to treat them normally so we can
      order uses of the same alias with different cv-qualification (79960).  */
   auto cso = make_temp_override (comparing_dependent_aliases);
-  if (!partial_order)
+  if (!comparing_for_partial_ordering)
     ++comparing_dependent_aliases;
 
   if (TREE_CODE (nt) == TREE_VEC || TREE_CODE (ot) == TREE_VEC)
@@ -9393,8 +9404,7 @@ template_args_equal (tree ot, tree nt, bool partial_order /* = false */)
 
 bool
 comp_template_args (tree oldargs, tree newargs,
-		    tree *oldarg_ptr /* = NULL */, tree *newarg_ptr /* = NULL */,
-		    bool partial_order /* = false */)
+		    tree *oldarg_ptr /* = NULL */, tree *newarg_ptr /* = NULL */)
 {
   if (oldargs == newargs)
     return true;
@@ -9410,7 +9420,7 @@ comp_template_args (tree oldargs, tree newargs,
       tree nt = TREE_VEC_ELT (newargs, i);
       tree ot = TREE_VEC_ELT (oldargs, i);
 
-      if (! template_args_equal (ot, nt, partial_order))
+      if (! template_args_equal (ot, nt))
 	{
 	  if (oldarg_ptr != NULL)
 	    *oldarg_ptr = ot;
@@ -9422,10 +9432,13 @@ comp_template_args (tree oldargs, tree newargs,
   return true;
 }
 
-inline bool
+static bool
 comp_template_args_porder (tree oargs, tree nargs)
 {
-  return comp_template_args (oargs, nargs, NULL, NULL, true);
+  ++comparing_for_partial_ordering;
+  bool equal = comp_template_args (oargs, nargs);
+  --comparing_for_partial_ordering;
+  return equal;
 }
 
 /* Implement a freelist interface for objects of type T.
@@ -28727,6 +28740,19 @@ any_template_arguments_need_structural_equality_p (tree args)
 		   mutated after the fact by duplicate_decls), so just require
 		   structural equality in this case (PR52830).  */
 		return true;
+	      else if (TYPE_P (arg)
+		       && TYPE_STRUCTURAL_EQUALITY_P (arg)
+		       && dependent_alias_template_spec_p (arg, nt_transparent))
+		/* Require structural equality for specializations written
+		   in terms of a dependent alias template specialization.  */
+		return true;
+	      else if (CLASS_TYPE_P (arg)
+		       && TYPE_TEMPLATE_INFO (arg)
+		       && TYPE_STRUCTURAL_EQUALITY_P (arg))
+		/* Require structural equality for specializations written
+		   in terms of a class template specialization that itself
+		   needs structural equality.  */
+		return true;
 	    }
 	}
     }

gcc/cp/tree.cc

@@ -4131,7 +4131,7 @@ cp_tree_equal (tree t1, tree t2)
     case TREE_VEC:
       /* These are template args.  Really we should be getting the
 	 caller to do this as it knows it to be true.  */
-      if (!comp_template_args (t1, t2, NULL, NULL, false))
+      if (!comp_template_args (t1, t2))
 	return false;
       return true;
 

gcc/testsuite/g++.dg/cpp0x/alias-decl-75a.C

@@ -0,0 +1,26 @@
+// PR c++/90679
+// A version of alias-decl-75.C where the specializations of the
+// complex alias template first_t are dependent.
+// { dg-do compile { target c++11 } }
+
+template<class T, class...>
+using first_t = T;
+
+template<class T>
+struct A;
+
+template<class T>
+struct traits;
+
+template<class T>
+struct traits<A<first_t<T, T&>>> {
+  static constexpr int value = 1;
+};
+
+template<class T>
+struct traits<A<first_t<const T, T&>>> {
+  static constexpr int value = 2;
+};
+
+static_assert(traits<A<int>>::value == 1, "");
+static_assert(traits<A<const int>>::value == 2, ""); // { dg-bogus "ambiguous" }

gcc/testsuite/g++.dg/cpp0x/alias-decl-75b.C

@@ -0,0 +1,26 @@
+// PR c++/90679
+// A version of alias-decl-75a.C where the alias template specialization
+// appears as a more deeply nested template argument.
+// { dg-do compile { target c++11 } }
+
+template<class T, class...>
+using first_t = T;
+
+template<class T>
+struct A;
+
+template<class T>
+struct traits;
+
+template<class T>
+struct traits<A<A<first_t<T, T&>>>> {
+  static constexpr int value = 1;
+};
+
+template<class T>
+struct traits<A<A<first_t<const T, T&>>>> {
+  static constexpr int value = 2;
+};
+
+static_assert(traits<A<A<int>>>::value == 1, "");
+static_assert(traits<A<A<const int>>>::value == 2, ""); // { dg-bogus "ambiguous" }