Re: [PATCH] c++: Fix up parameter pack diagnostics on xobj vs. varargs functions [PR113802]

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Feb 16, 2024 at 10:47:47PM +0100, Jakub Jelinek wrote:
> The following patch works.

Or yet another option would be instead of (sometimes) clearing
declarator->parameter_pack_p when we diagnose this bug for error
recovery ignore the this specifier.
With the following patch (testsuite patch remains the same),
I get excess errors though:
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:30:25: error: expansion pattern 'Selves' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:42:26: error: expansion pattern 'Selves' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:56:26: error: expansion pattern 'Selves&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:68:27: error: expansion pattern 'Selves&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:82:27: error: expansion pattern 'Selves&&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:94:28: error: expansion pattern 'Selves&&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:108:32: error: expansion pattern 'const Selves&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:120:33: error: expansion pattern 'const Selves&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:134:33: error: expansion pattern 'const Selves&&' contains no parameter packs
/usr/src/gcc/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C:146:34: error: expansion pattern 'const Selves&&' contains no parameter packs
though, that is e.g. on
struct S0 {
  template<typename Selves>
  void g(this Selves... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
}
where such an extra error would have been emitted if the this keyword was
omitted.

--- gcc/cp/parser.cc.jj	2024-02-16 17:38:27.802845433 +0100
+++ gcc/cp/parser.cc	2024-02-16 23:08:40.835437740 +0100
@@ -25734,22 +25734,6 @@ cp_parser_parameter_declaration (cp_pars
       decl_specifiers.locations[ds_this] = 0;
     }
 
-  if (xobj_param_p
-      && ((declarator && declarator->parameter_pack_p)
-	  || cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS)))
-    {
-      location_t xobj_param
-	= make_location (decl_specifiers.locations[ds_this],
-			 decl_spec_token_start->location,
-			 input_location);
-      error_at (xobj_param,
-		"an explicit object parameter cannot "
-		"be a function parameter pack");
-      /* Suppress errors that occur down the line.  */
-      if (declarator)
-	declarator->parameter_pack_p = false;
-    }
-
   /* If a function parameter pack was specified and an implicit template
      parameter was introduced during cp_parser_parameter_declaration,
      change any implicit parameters introduced into packs.  */
@@ -25762,9 +25746,10 @@ cp_parser_parameter_declaration (cp_pars
 	(INNERMOST_TEMPLATE_PARMS (current_template_parms));
 
       if (latest_template_parm_idx != template_parm_idx)
-	decl_specifiers.type = convert_generic_types_to_packs
-	  (decl_specifiers.type,
-	   template_parm_idx, latest_template_parm_idx);
+	decl_specifiers.type
+	  = convert_generic_types_to_packs (decl_specifiers.type,
+					    template_parm_idx,
+					    latest_template_parm_idx);
     }
 
   if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
@@ -25794,6 +25779,21 @@ cp_parser_parameter_declaration (cp_pars
 	}
     }
 
+  if (xobj_param_p
+      && (declarator ? declarator->parameter_pack_p
+		     : PACK_EXPANSION_P (decl_specifiers.type)))
+    {
+      location_t xobj_param
+	= make_location (decl_specifiers.locations[ds_this],
+			 decl_spec_token_start->location,
+			 input_location);
+      error_at (xobj_param,
+		"an explicit object parameter cannot "
+		"be a function parameter pack");
+      xobj_param_p = false;
+      decl_specifiers.locations[ds_this] = 0;
+    }
+
   /* The restriction on defining new types applies only to the type
      of the parameter, not to the default argument.  */
   parser->type_definition_forbidden_message = saved_message;

	Jakub