c++: Implement C++23 P0849R8 - auto(x) [PR103049]

This patch implements P0849R8 which allows auto in a functional cast,
the result of which is a prvalue.

[expr.type.conv]/1 says that the type is determined by placeholder type
deduction.  We only accept 'auto', not 'decltype(auto)' -- that the
type shall be auto comes from [dcl.type.auto.deduct].  Therefore the
rules are like for [temp.deduct.call], deducing template arguments from
a function call, so the result type will never be a reference, and we
decay arrays/functions.

	PR c++/103049

gcc/cp/ChangeLog:

	* semantics.c (finish_compound_literal): Accept C++23 auto{x}.
	* typeck2.c (build_functional_cast_1): Accept C++23 auto(x).

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/auto25.C: Adjust dg-error.
	* g++.dg/cpp0x/auto9.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-3.C: Likewise.
	* g++.dg/cpp23/auto-fncast1.C: New test.
	* g++.dg/cpp23/auto-fncast2.C: New test.
	* g++.dg/cpp23/auto-fncast3.C: New test.
	* g++.dg/cpp23/auto-fncast4.C: New test.
	* g++.dg/cpp23/auto-fncast5.C: New test.
	* g++.dg/cpp23/auto-fncast6.C: New test.

1 files changed, 18 insertions, 8 deletions

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index e98fbf7..3fb651a 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -2201,19 +2201,29 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 
   if (tree anode = type_uses_auto (type))
     {
-      if (!CLASS_PLACEHOLDER_TEMPLATE (anode))
+      tree init;
+      if (CLASS_PLACEHOLDER_TEMPLATE (anode))
+	init = parms;
+      /* C++23 auto(x).  */
+      else if (!AUTO_IS_DECLTYPE (anode)
+	       && list_length (parms) == 1)
 	{
-	  if (complain & tf_error)
-	    error_at (loc, "invalid use of %qT", anode);
-	  return error_mark_node;
+	  init = TREE_VALUE (parms);
+	  if (cxx_dialect < cxx23)
+	    pedwarn (loc, OPT_Wc__23_extensions,
+		     "%<auto(x)%> only available with "
+		     "%<-std=c++2b%> or %<-std=gnu++2b%>");
 	}
       else
 	{
-	  type = do_auto_deduction (type, parms, anode, complain,
-				    adc_variable_type);
-	  if (type == error_mark_node)
-	    return error_mark_node;
+	  if (complain & tf_error)
+	    error_at (loc, "invalid use of %qT", anode);
+	  return error_mark_node;
 	}
+      type = do_auto_deduction (type, init, anode, complain,
+				adc_variable_type);
+      if (type == error_mark_node)
+	return error_mark_node;
     }
 
   if (processing_template_decl)