Failing to parse wirh clang-20 libc++

[mingodad]

When trying to parse using the clan-20 libc++ headers we get the error shown bellow (it parses fine with clang-19 libc++):

/usr/bin/time cppcxx0 -fsyntax-only -std=c++23 -toolchain linux -I clang-20/include/c++/v1 -I clang-20/include/x86_64-unknown-linux-gnu/c++/v1 -I../../cplusplus-parser/src/parser -I../../cplusplus-parser/build-zig/_deps/nlohmann_json-src/single_include -I../../cplusplus-parser/build-zig/_deps/utfcpp-src/source -DCXX_VERSION="1.1.27" -DCXX_NO_FLATBUFFERS
../../cplusplus-parser/src/parser/cxx/external_name_encoder.cc
clang-20/include/c++/v1/__functional/not_fn.h:67:17: error: expected an expression
  if constexpr (using _Ty = decltype(_Fn); is_pointer_v<_Ty> || is_member_pointer_v<_Ty>)
                ^

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___FUNCTIONAL_NOT_FN_H
#define _LIBCPP___FUNCTIONAL_NOT_FN_H

#include <__config>
#include <__functional/invoke.h>
#include <__functional/perfect_forward.h>
#include <__type_traits/decay.h>
#include <__type_traits/enable_if.h>
#include <__type_traits/is_constructible.h>
#include <__type_traits/is_member_pointer.h>
#include <__type_traits/is_pointer.h>
#include <__utility/forward.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 17

struct __not_fn_op {
  template <class... _Args>
  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 auto operator()(_Args&&... __args) const
      noexcept(noexcept(!std::invoke(std::forward<_Args>(__args)...)))
          -> decltype(!std::invoke(std::forward<_Args>(__args)...)) {
    return !std::invoke(std::forward<_Args>(__args)...);
  }
};

template <class _Fn>
struct __not_fn_t : __perfect_forward<__not_fn_op, _Fn> {
  using __perfect_forward<__not_fn_op, _Fn>::__perfect_forward;
};

template <class _Fn,
          class = enable_if_t< is_constructible_v<decay_t<_Fn>, _Fn> && is_move_constructible_v<decay_t<_Fn>> >>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 auto not_fn(_Fn&& __f) {
  return __not_fn_t<decay_t<_Fn>>(std::forward<_Fn>(__f));
}

#endif // _LIBCPP_STD_VER >= 17

#if _LIBCPP_STD_VER >= 26

template <auto _Fn>
struct __nttp_not_fn_t {
  template <class... _Args>
  [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Args&&... __args) const
      noexcept(noexcept(!std::invoke(_Fn, std::forward<_Args>(__args)...)))
          -> decltype(!std::invoke(_Fn, std::forward<_Args>(__args)...)) {
    return !std::invoke(_Fn, std::forward<_Args>(__args)...);
  }
};

template <auto _Fn>
[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI constexpr auto not_fn() noexcept {
  if constexpr (using _Ty = decltype(_Fn); is_pointer_v<_Ty> || is_member_pointer_v<_Ty>)
    static_assert(_Fn != nullptr, "f cannot be equal to nullptr");
  return __nttp_not_fn_t<_Fn>();
}

#endif // _LIBCPP_STD_VER >= 26

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___FUNCTIONAL_NOT_FN_H

[robertoraggi]

is this on linux?

[robertoraggi]

Nevermind, I was able to reproduce it. This is related to allow aliases in init statements

https://eel.is/c++draft/stmt.pre#nt:init-statement