c++: Fix up calls to static operator() or operator[] [PR107624]

One thing that doesn't work properly is the same problem as I've filed
yesterday for static operator() - PR107624 - that side-effects of
the postfix-expression on which the call or subscript operator are
applied are thrown away, I assume we have to add them into COMPOUND_EXPR
somewhere after we find out that the we've chosen a static member function
operator.

On Mon, Nov 14, 2022 at 06:29:44PM -0500, Jason Merrill wrote:
> Indeed.  The code in build_new_method_call for this case has the comment
>
>               /* In an expression of the form `a->f()' where `f' turns
>                  out to be a static member function, `a' is
>                  none-the-less evaluated.  */

Had to tweak 3 spots for this.  Furthermore, found that if in non-pedantic
C++20 compilation static operator[] is accepted, we required that it has 2
arguments, I think it is better to require exactly one because that case
is the only one that will actually work in C++20 and older.

2022-11-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107624
	* call.cc (keep_unused_object_arg): New function.
	(build_op_call): Use it.
	(build_op_subscript): Likewise.
	(build_new_op): Similarly for ARRAY_REF.
	(build_new_method_call): Use it.
	* decl.cc (grok_op_properties): For C++20 and earlier, if operator[]
	is static member function, require exactly one parameter rather than
	exactly two parameters.

	* g++.dg/cpp23/static-operator-call4.C: New test.
	* g++.dg/cpp23/subscript10.C: New test.
	* g++.dg/cpp23/subscript11.C: New test.