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

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.