c++: Reject alias CTAD in C++17 [PR99008]

Here, in C++17 mode, we only pedwarn about the use of alias CTAD and
then later ICE from alias_ctad_tweaks when attempting to constrain
the guides.  Since the construction of the guides of an alias template
effectively relies on concepts, we shouldn't be permissive about alias
CTAD in C++17 mode, so this patch turns the pertinent pedwarn in
do_class_deduction into an error.

In order to get a consistent diagnostic for B() vs the other forms in
the added testcase, I had to remove the special handling of CTAD with
empty initializer in build_functional_cast_1 so that we always pass
'complain' to do_auto_deduction.

gcc/cp/ChangeLog:

	PR c++/99008
	* pt.c (do_class_deduction): Reject alias CTAD in C++17 mode
	rather than issuing a pedwarn.
	* typeck2.c (build_functional_cast_1): Handle CTAD uniformly
	for consistent diagnostics.

gcc/testsuite/ChangeLog:

	PR c++/99008
	* g++.dg/parse/template2.C: Adjust expected diagnostic.
	* g++.dg/template/error8.C: Likewise.
	* g++.dg/cpp1z/class-deduction84.C: New test.

1 files changed, 3 insertions, 14 deletions

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index a58d397..4e9632f 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -2197,24 +2197,13 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 	    error_at (loc, "invalid use of %qT", anode);
 	  return error_mark_node;
 	}
-      else if (!parms)
+      else
 	{
-	  /* Even if there are no parameters, we might be able to deduce from
-	     default template arguments.  Pass TF_NONE so that we don't
-	     generate redundant diagnostics.  */
-	  type = do_auto_deduction (type, parms, anode, tf_none,
+	  type = do_auto_deduction (type, parms, anode, complain,
 				    adc_variable_type);
 	  if (type == error_mark_node)
-	    {
-	      if (complain & tf_error)
-		error_at (loc, "cannot deduce template arguments "
-			  "for %qT from %<()%>", anode);
-	      return error_mark_node;
-	    }
+	    return error_mark_node;
 	}
-      else
-	type = do_auto_deduction (type, parms, anode, complain,
-				  adc_variable_type);
     }
 
   if (processing_template_decl)