Created attachment 53565 [details]
gcc13-pr106651-wip.patch

WIP patch which has parts of the changes implemented.
What is missing is [over.match.best.general]/1 and [over.best.ics.general]
changes and the library side, which means e.g. for the testcase from the paper:
struct less {
    static constexpr auto operator()(int i, int j) -> bool {
        return i < j;
    }

    using P = bool(*)(int, int);
    operator P() const { return operator(); }
};

static_assert(less{}(1, 2));
we ICE.
Testcase I've been playing with that compiles now:
template <typename T>
struct S
{
  static constexpr bool operator() (T const& x, T const& y) { return x < y; };
};

template <typename T>
void
bar (T &x)
{
  x (1, 2);
}

void
foo ()
{
  auto a = [](int x, int y) static { return x + y; };
  bar (*a);
  auto b = []<typename T, typename U>(T x, U y) static { return x + y; };
  b (1, 2L);
  S<int> s;
  s(1, 2);
}

Created attachment 53571 [details]
gcc13-pr106651.patch

Full untested patch.

The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:

https://gcc.gnu.org/g:303976a6076f2839354702fd2caa049fa7cbbdc2

commit r13-2892-g303976a6076f2839354702fd2caa049fa7cbbdc2
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Sep 27 08:36:28 2022 +0200

    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.

The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:614e5696d730a65998ff5b0373f905795a758dd6

commit r13-2897-g614e5696d730a65998ff5b0373f905795a758dd6
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Sep 27 11:25:51 2022 +0100

    libstdc++: Adjust deduction guides for static operator() [PR106651]
    
    Adjust the deduction guides for std::function and std::packaged_task to
    work with static call operators. This finishes the implementation of
    P1169R4 for C++23.
    
    libstdc++-v3/ChangeLog:
    
            PR c++/106651
            * include/bits/std_function.h (__function_guide_t): New alias
            template.
            [__cpp_static_call_operator] (__function_guide_static_helper):
            New class template.
            (function): Use __function_guide_t in deduction guide.
            * include/std/future (packaged_task): Use __function_guide_t in
            deduction guide.
            * testsuite/20_util/function/cons/deduction_c++23.cc: New test.
            * testsuite/30_threads/packaged_task/cons/deduction_c++23.cc:
            New test.

This is complete now.

The trunk branch has been updated by Jason Merrill <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:4f181f9c7ee3efc509d185fdfda33be9018f1611

commit r13-6533-g4f181f9c7ee3efc509d185fdfda33be9018f1611
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Mar 6 21:36:28 2023 -0500

    c++: static lambda tsubst [PR108526]
    
    A missed piece of the patch for static operator(): in tsubst_function_decl,
    we don't want to replace the first parameter with a new closure pointer if
    operator() is static.
    
            PR c++/108526
            PR c++/106651
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_function_decl): Don't replace the closure
            parameter if DECL_STATIC_FUNCTION_P.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp23/static-operator-call5.C: Pass -g.