Implement P1814R0, CTAD for alias templates.

This patch implements C++20 class template argument deduction for alias
templates, which works by a moderately arcane transformation of the
deduction guides for the underlying class template.  When implementing it,
it seemed that I could simplify the rules in the draft a bit and get
essentially the same effect; I'll be emailing the committee to that effect
soon.

gcc/cp/
	* pt.c (rewrite_tparm_list): Factor out of build_deduction_guide.
	(maybe_aggr_guide): Check for copy-init here.
	(alias_ctad_tweaks, deduction_guides_for): New.
	(ctor_deduction_guides_for): Factor out of do_class_deduction.
	(ctad_template_p): New.
	* parser.c (cp_parser_simple_type_specifier): Use it.
	* constraint.cc (append_constraint): New.
gcc/c-family/
	* c-cppbuiltin.c (c_cpp_builtins): Update __cpp_deduction_guides.

From-SVN: r278786

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 0d1c27a..533277a 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1116,6 +1116,25 @@ build_constraints (tree tr, tree dr)
   return (tree)ci;
 }
 
+/* Add constraint RHS to the end of CONSTRAINT_INFO ci.  */
+
+tree
+append_constraint (tree ci, tree rhs)
+{
+  tree tr = ci ? CI_TEMPLATE_REQS (ci) : NULL_TREE;
+  tree dr = ci ? CI_DECLARATOR_REQS (ci) : NULL_TREE;
+  dr = combine_constraint_expressions (dr, rhs);
+  if (ci)
+    {
+      CI_DECLARATOR_REQS (ci) = dr;
+      tree ac = combine_constraint_expressions (tr, dr);
+      CI_ASSOCIATED_CONSTRAINTS (ci) = ac;
+    }
+  else
+    ci = build_constraints (tr, dr);
+  return ci;
+}
+
 /* A mapping from declarations to constraint information.  */
 
 static GTY ((cache)) decl_tree_cache_map *decl_constraints;