c++: Handle auto(x) in parameter-declaration-clause [PR103401]

In C++23, auto(x) is valid, so decltype(auto(x)) should also be valid,
so

  void f(decltype(auto(0)));

should be just as

  void f(int);

but currently, everytime we see 'auto' in a parameter-declaration-clause,
we try to synthesize_implicit_template_parm for it, creating a new template
parameter list.  The code above actually has us calling s_i_t_p twice;
once from cp_parser_decltype_expr -> cp_parser_postfix_expression which
fails and then again from cp_parser_decltype_expr -> cp_parser_expression.
So it looks like we have f<auto, auto> and we accept ill-formed code.

This shows that we need to be more careful about synthesizing the
implicit template parameter.  [dcl.spec.auto.general] says that "A
placeholder-type-specifier of the form type-constraintopt auto can be
used as a decl-specifier of the decl-specifier-seq of a
parameter-declaration of a function declaration or lambda-expression..."
so this patch turns off auto_is_... after we've parsed the decl-specifier-seq.

That doesn't quite cut it yet though, because we also need to handle an
auto nested in the decl-specifier:

  void f(decltype(new auto{0}));

therefore the cp_parser_decltype change.

To accept "sizeof(auto{10})", the cp_parser_type_id_1 hunk only gives a
hard error when we're not parsing tentatively.

The cp_parser_parameter_declaration hunk broke lambda-generic-85713-2.C but
I think the error we issue with this patch is in fact correct, and clang++
agrees.

The r11-1913 change is OK: we need to make sure that we see '(auto)' after
decltype to go ahead with 'decltype(auto)'.

	PR c++/103401

gcc/cp/ChangeLog:

	* parser.c (cp_parser_decltype): Clear
	auto_is_implicit_function_template_parm_p.
	(cp_parser_type_id_1): Give errors only when !cp_parser_simulate_error.
	(cp_parser_parameter_declaration): Clear
	auto_is_implicit_function_template_parm_p after parsing the
	decl-specifier-seq.
	(cp_parser_sizeof_operand): Clear
	auto_is_implicit_function_template_parm_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-85713-2.C: Add dg-error.
	* g++.dg/cpp1y/pr60054.C: Adjust dg-error.
	* g++.dg/cpp1y/pr60332.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979-3.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84979.C: Likewise.
	* g++.dg/cpp23/auto-fncast7.C: New test.
	* g++.dg/cpp23/auto-fncast8.C: New test.
	* g++.dg/cpp23/auto-fncast9.C: New test.

1 files changed, 30 insertions, 8 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 6f273bf..de464afd 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -16432,6 +16432,16 @@ cp_parser_decltype (cp_parser *parser)
 	= parser->greater_than_is_operator_p;
       parser->greater_than_is_operator_p = true;
 
+      /* Don't synthesize an implicit template type parameter here.  This
+	 could happen with C++23 code like
+
+	   void f(decltype(new auto{0}));
+
+	 where we want to deduce the auto right away so that the parameter
+	 is of type 'int *'.  */
+      auto cleanup = make_temp_override
+	(parser->auto_is_implicit_function_template_parm_p, false);
+
       /* Do not actually evaluate the expression.  */
       ++cp_unevaluated_operand;
 
@@ -24144,22 +24154,22 @@ cp_parser_type_id_1 (cp_parser *parser, cp_parser_flags flags,
 	  /* OK */;
 	else
 	  {
-	    location_t loc = type_specifier_seq.locations[ds_type_spec];
-	    if (tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
+	    if (!cp_parser_simulate_error (parser))
 	      {
-		if (!cp_parser_simulate_error (parser))
+		location_t loc = type_specifier_seq.locations[ds_type_spec];
+		if (tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
 		  {
 		    error_at (loc, "missing template arguments after %qT",
 			      auto_node);
 		    inform (DECL_SOURCE_LOCATION (tmpl), "%qD declared here",
 			    tmpl);
 		  }
+		else if (parser->in_template_argument_list_p)
+		  error_at (loc, "%qT not permitted in template argument",
+			    auto_node);
+		else
+		  error_at (loc, "invalid use of %qT", auto_node);
 	      }
-	    else if (parser->in_template_argument_list_p)
-	      error_at (loc, "%qT not permitted in template argument",
-			auto_node);
-	    else
-	      error_at (loc, "invalid use of %qT", auto_node);
 	    return error_mark_node;
 	  }
       }
@@ -24668,6 +24678,15 @@ cp_parser_parameter_declaration (cp_parser *parser,
 				&decl_specifiers,
 				&declares_class_or_enum);
 
+  /* [dcl.spec.auto.general]: "A placeholder-type-specifier of the form
+     type-constraint opt auto can be used as a decl-specifier of the
+     decl-specifier-seq of a parameter-declaration of a function declaration
+     or lambda-expression..." but we must not synthesize an implicit template
+     type parameter in its declarator.  That is, in "void f(auto[auto{10}]);"
+     we want to synthesize only the first auto.  */
+  auto cleanup = make_temp_override
+    (parser->auto_is_implicit_function_template_parm_p, false);
+
   /* Complain about missing 'typename' or other invalid type names.  */
   if (!decl_specifiers.any_type_specifiers_p
       && cp_parser_parse_and_diagnose_invalid_type_name (parser))
@@ -32369,6 +32388,9 @@ cp_parser_sizeof_operand (cp_parser* parser, enum rid keyword)
     = parser->non_integral_constant_expression_p;
   parser->integral_constant_expression_p = false;
 
+  auto cleanup = make_temp_override
+    (parser->auto_is_implicit_function_template_parm_p, false);
+
   /* Do not actually evaluate the expression.  */
   ++cp_unevaluated_operand;
   ++c_inhibit_evaluation_warnings;