c++: Implement C++23 P1169R4 - static operator() [PR106651]

c++: Implement C++23 P1169R4 - static operator() [PR106651]

The following patch attempts to implement C++23 P1169R4 - static operator()
paper's compiler side (there is some small library side too not implemented
yet).  This allows static members as user operator() declarations and
static specifier on lambdas without lambda capture.
The synthetized conversion operator changes for static lambdas as it can just
return the operator() static method address, doesn't need to create a thunk
for it.
The change in call.cc (joust) is to avoid ICEs because we assumed that len
could be different only if both candidates are direct calls but it can be
one direct and one indirect call, and to implement the
[over.match.best.general]/1 and [over.best.ics.general] changes from
the paper (implemented always as Jason is sure it doesn't make a difference
in C++20 and earlier unless static member function operator() or
static lambda which we accept with pedwarn in earlier standards too appears
and my testing confirmed that).

2022-09-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106651
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_static_call_operator=202207L for C++23.
gcc/cp/
	* cp-tree.h (LAMBDA_EXPR_STATIC_P): Implement C++23
	P1169R4 - static operator().  Define.
	* parser.cc (CP_PARSER_FLAGS_ONLY_MUTABLE_OR_CONSTEXPR): Document
	that it also allows static.
	(cp_parser_lambda_declarator_opt): Handle static lambda specifier.
	(cp_parser_decl_specifier_seq): Allow RID_STATIC for
	CP_PARSER_FLAGS_ONLY_MUTABLE_OR_CONSTEXPR.
	* decl.cc (grok_op_properties): If operator() isn't a method,
	use a different error wording, if it is static member function,
	allow it (for C++20 and older with a pedwarn unless it is
	a lambda function or template instantiation).
	* call.cc (joust): Don't ICE if one candidate is static member
	function and the other is an indirect call.  If the parameter
	conversion on the other candidate is user defined conversion,
	ellipsis or bad conversion, make static member function candidate
	a winner for that parameter.
	* lambda.cc (maybe_add_lambda_conv_op): Handle static lambdas.
	* error.cc (dump_lambda_function): Print static for static lambdas.
gcc/testsuite/
	* g++.dg/template/error30.C: Adjust expected diagnostics.
	* g++.dg/cpp1z/constexpr-lambda13.C: Likewise.
	* g++.dg/cpp23/feat-cxx2b.C: Test __cpp_static_call_operator.
	* g++.dg/cpp23/static-operator-call1.C: New test.
	* g++.dg/cpp23/static-operator-call2.C: New test.
	* g++.old-deja/g++.jason/operator.C: Adjust expected diagnostics.