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

The simple presence of ellipsis as next token after the parameter
declaration doesn't imply it is a parameter pack, it sometimes is, e.g.
if its type is a pack, but sometimes is not and in that case it acts
the same as if the next tokens were , ... instead of just ...
The xobj param cannot be a function parameter pack though treats both
the declarator->parameter_pack_p and token->type == CPP_ELLIPSIS as
sufficient conditions for the error.  The conditions for CPP_ELLIPSIS
are done a little bit later in the same function and complex enough that
IMHO shouldn't be repeated, on the other side for the
declarator->parameter_pack_p case we clear that flag for xobj params
for error recovery reasons.

This patch just moves the diagnostics later (after the CPP_ELLIPSIS handling)
and changes the error recovery behavior by pretending the this specifier
didn't appear if an error is reported.

2024-03-08  Jakub Jelinek  <jakub@redhat.com>

	PR c++/113802
	* parser.cc (cp_parser_parameter_declaration): Move the xobj_param_p
	pack diagnostics after ellipsis handling and if an error is reported,
	pretend this specifier didn't appear.  Formatting fix.

	* g++.dg/cpp23/explicit-obj-diagnostics3.C (S0, S1, S2, S3, S4): Don't
	expect any diagnostics on f and fd member function templates, add
	similar templates with ...Selves instead of Selves as k and kd and
	expect diagnostics for those.  Expect extra diagnostics in error
	recovery for g and gd member function templates.

2 files changed, 69 insertions, 30 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e32acfc..bc3aa9d 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -25734,22 +25734,6 @@ cp_parser_parameter_declaration (cp_parser *parser,
       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_parser *parser,
 	(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_parser *parser,
 	}
     }
 
+  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;
diff --git a/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C b/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C
index 304cf02..f6a892e 100644
--- a/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C
+++ b/gcc/testsuite/g++.dg/cpp23/explicit-obj-diagnostics3.C
@@ -24,101 +24,140 @@ void S::f12(this S s = {}) {} // { dg-error "an explicit object parameter may no
 
 struct S0 {
   template<typename Selves>
-  void f(this Selves...) {} // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void f(this Selves...) {}
 
   template<typename Selves>
   void g(this Selves... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				    // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void h(this auto...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void j(this auto... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 
+  template<typename ...Selves>
+  void k(this Selves...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
   template<typename Selves>
-  void fd(this Selves...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void fd(this Selves...);
 
   template<typename Selves>
   void gd(this Selves... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				   // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void hd(this auto...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void jd(this auto... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
+  template<typename ...Selves>
+  void kd(this Selves...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 };
 
 struct S1 {
   template<typename Selves>
-  void f(this Selves&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void f(this Selves&...) {}
 
   template<typename Selves>
   void g(this Selves&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				     // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void h(this auto&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void j(this auto&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 
+  template<typename ...Selves>
+  void k(this Selves&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
   template<typename Selves>
-  void fd(this Selves&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void fd(this Selves&...);
 
   template<typename Selves>
   void gd(this Selves&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				    // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void hd(this auto&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void jd(this auto&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
+  template<typename ...Selves>
+  void kd(this Selves&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 };
 
 struct S2 {
   template<typename Selves>
-  void f(this Selves&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void f(this Selves&&...) {}
 
   template<typename Selves>
   void g(this Selves&&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				      // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void h(this auto&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void j(this auto&&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 
+  template<typename ...Selves>
+  void k(this Selves&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
   template<typename Selves>
-  void fd(this Selves&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void fd(this Selves&&...);
 
   template<typename Selves>
   void gd(this Selves&&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+				     // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void hd(this auto&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void jd(this auto&&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
+  template<typename ...Selves>
+  void kd(this Selves&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 };
 
 struct S3 {
   template<typename Selves>
-  void f(this Selves const&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void f(this Selves const&...) {}
 
   template<typename Selves>
   void g(this Selves const&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+					   // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void h(this auto const&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void j(this auto const&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 
+  template<typename ...Selves>
+  void k(this Selves const&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
   template<typename Selves>
-  void fd(this Selves const&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void fd(this Selves const&...);
 
   template<typename Selves>
   void gd(this Selves const&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+					  // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void hd(this auto const&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void jd(this auto const&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
+  template<typename ...Selves>
+  void kd(this Selves const&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 };
 
 struct S4 {
   template<typename Selves>
-  void f(this Selves const&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void f(this Selves const&&...) {}
 
   template<typename Selves>
   void g(this Selves const&&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+					    // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void h(this auto const&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void j(this auto const&&... selves) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
 
+  template<typename ...Selves>
+  void k(this Selves const&&...) {}  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+
   template<typename Selves>
-  void fd(this Selves const&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+  void fd(this Selves const&&...);
 
   template<typename Selves>
   void gd(this Selves const&&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+					   // { dg-error "contains no parameter packs" "" { target *-*-* } .-1 }
 
   void hd(this auto const&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
   void jd(this auto const&&... selves);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
-};
 
+  template<typename ...Selves>
+  void kd(this Selves const&&...);  // { dg-error "an explicit object parameter cannot be a function parameter pack" }
+};