Implement P0091R2, Template argument deduction for class templates.

* parser.c (cp_parser_simple_type_specifier): Parse class placeholder.
Use the location of the beginning of the type-specifier.
(cp_parser_init_declarator): Parse deduction guide.
(cp_parser_diagnose_invalid_type_name): Mention class deduction.
(cp_parser_type_id_1): Don't accept class placeholder as template arg.
* cp-tree.h (CLASS_PLACEHOLDER_TEMPLATE): New.
* decl.c (grokdeclarator): Check for uninitialized auto here.
(start_decl_1): Not here.
(cp_finish_decl): Or here.  Don't collapse a list when doing
class deduction.
(grokfndecl): Check deduction guide scope and body.
* error.c (dump_decl, dump_function_decl, dump_function_name):
Handle deduction guides.
* pt.c (make_template_placeholder, do_class_deduction): New.
(build_deduction_guide, rewrite_template_parm): New.
(dguide_name, dguide_name_p, deduction_guide_p): New.
(do_auto_deduction): Call do_class_deduction.
(splice_late_return_type, is_auto): Handle class placeholders.
(template_parms_level_to_args): Split from template_parms_to_args.
(tsubst_template_parms_level): Split from tsubst_template_parms.
* typeck2.c (build_functional_cast): Handle class placeholder.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@240756 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 16 insertions, 4 deletions

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index 6e226856edc..a063ea34392 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -1952,11 +1952,23 @@ build_functional_cast (tree exp, tree parms, tsubst_flags_t complain)
       return error_mark_node;
     }
 
-  if (type_uses_auto (type))
+  if (tree anode = type_uses_auto (type))
     {
-      if (complain & tf_error)
-	error ("invalid use of %<auto%>");
-      return error_mark_node;
+      if (!CLASS_PLACEHOLDER_TEMPLATE (anode))
+	{
+	  if (complain & tf_error)
+	    error ("invalid use of %qT", anode);
+	  return error_mark_node;
+	}
+      else if (!parms)
+	{
+	  if (complain & tf_error)
+	    error ("cannot deduce template arguments for %qT from ()", anode);
+	  return error_mark_node;
+	}
+      else
+	type = do_auto_deduction (type, parms, anode, complain,
+				  adc_variable_type);
     }
 
   if (processing_template_decl)