Ranges pipe syntax broken after commit #c1086532d4d5 when using std module.

[stripe2933]

The following code

#include <print>
#include <ranges>

int main(){
    using namespace std::string_view_literals;
    std::println("{}", "abcde"sv | std::views::transform([](char c) { return c - 'a'; }));
    
}

works correctly (godbolt link) in Clang trunk. However, when changing the header inclusion to import std;, then the code will emit error:

main.cpp:5:34: error: invalid operands to binary expression ('basic_string_view<char>' and '__range_adaptor_closure_t<__bind_back_t<__fn, tuple<(lambda at main.cpp:5:58)>>>' (aka 'std::__1::ranges::__range_adaptor_closure_t<std::__bind_back_t<std::__1::ranges::views::__transform::__fn, std::__1::tuple<(lambda at main.cpp:5:58)>>>'))
    5 |     std::println("{}", "abcde"sv | std::views::transform([](char c) { return c - 'a'; }));
      |                        ~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
llvm-project/build/include/c++/v1/cstddef:73:45: note: candidate function not viable: no known conversion from 'basic_string_view<char>' to 'byte' for 1st argument
   73 | _LIBCPP_HIDE_FROM_ABI inline constexpr byte operator|(byte __lhs, byte __rhs) noexcept {
      |                                             ^         ~~~~~~~~~~
llvm-project/build/include/c++/v1/__charconv/chars_format.h:34:53: note: candidate function not viable: no known conversion from 'basic_string_view<char>' to 'chars_format' for 1st argument
   34 | inline _LIBCPP_HIDE_FROM_ABI constexpr chars_format operator|(chars_format __x, chars_format __y) {
      |                                                     ^         ~~~~~~~~~~~~~~~~
llvm-project/build/include/c++/v1/future:433:55: note: candidate function not viable: no known conversion from 'basic_string_view<char>' to 'launch' for 1st argument
  433 | inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR launch operator|(launch __x, launch __y) {
      |                                                       ^         ~~~~~~~~~~
llvm-project/build/include/c++/v1/bitset:929:1: note: candidate template ignored: could not match 'bitset' against 'basic_string_view'
  929 | operator|(const bitset<_Size>& __x, const bitset<_Size>& __y) _NOEXCEPT {
      | ^
llvm-project/build/include/c++/v1/valarray:2858:1: note: candidate template ignored: requirement '__is_val_expr<std::__1::basic_string_view<char, std::__1::char_traits<char>>>::value' was not satisfied [with _Expr1 = basic_string_view<char>, _Expr2 = __range_adaptor_closure_t<__bind_back_t<__fn, tuple<(lambda at main.cpp:5:58)>>>]
 2858 | operator|(const _Expr1& __x, const _Expr2& __y) {
      | ^
llvm-project/build/include/c++/v1/valarray:2867:5: note: candidate template ignored: requirement '__is_val_expr<std::__1::basic_string_view<char, std::__1::char_traits<char>>>::value' was not satisfied [with _Expr = basic_string_view<char>]
 2867 |     operator|(const _Expr& __x, const typename _Expr::value_type& __y) {
      |     ^
llvm-project/build/include/c++/v1/valarray:2876:5: note: candidate template ignored: requirement '__is_val_expr<std::__1::ranges::__range_adaptor_closure_t<std::__bind_back_t<std::__1::ranges::views::__transform::__fn, std::__1::tuple<(lambda at main.cpp:5:58)>>>>::value' was not satisfied [with _Expr = __range_adaptor_closure_t<__bind_back_t<__fn, tuple<(lambda at main.cpp:5:58)>>>]
 2876 |     operator|(const typename _Expr::value_type& __x, const _Expr& __y) {
      |     ^
1 error generated.

Compiler: Clang 18.1.4, Libc++: trunk.

I followed the standard library module instruction documented in libc++ document.

I noticed that the commits before c1086532d4d5, which corresponds to the PR #89148 does not print this error. It seems that the PR is problem.

[ldionne]

@stripe2933 Is this still an issue after #89793?

CC @xiaoyang-sde

[xiaoyang-sde]

@stripe2933 Is this still an issue after #89793?

CC @xiaoyang-sde

I tested the code with clang 18.1.4 and libc++ trunk. It's still an issue after #89793. It seems that ADL can't find the operator| in the std::ranges namespace when using import std. The issue is solved when I moved the operator| back into std::ranges::__range_adaptor_closure:

template <class _Tp>
  requires is_class_v<_Tp> && same_as<_Tp, remove_cv_t<_Tp>>
struct __range_adaptor_closure {
  template <ranges::range _Range, _RangeAdaptorClosure _Closure>
    requires same_as<_Tp, remove_cvref_t<_Closure>> && invocable<_Closure, _Range>
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr decltype(auto)
  operator|(_Range&& __range, _Closure&& __closure) noexcept(is_nothrow_invocable_v<_Closure, _Range>) {
    return std::invoke(std::forward<_Closure>(__closure), std::forward<_Range>(__range));
  }

  template <_RangeAdaptorClosure _Closure, _RangeAdaptorClosure _OtherClosure>
    requires same_as<_Tp, remove_cvref_t<_Closure>> && constructible_from<decay_t<_Closure>, _Closure> &&
             constructible_from<decay_t<_OtherClosure>, _OtherClosure>
  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI friend constexpr auto operator|(_Closure&& __c1, _OtherClosure&& __c2) noexcept(
      is_nothrow_constructible_v<decay_t<_Closure>, _Closure> &&
      is_nothrow_constructible_v<decay_t<_OtherClosure>, _OtherClosure>) {
    return __range_adaptor_closure_t(std::__compose(std::forward<_OtherClosure>(__c2), std::forward<_Closure>(__c1)));
  }
};

[ldionne]

@ChuanqiXu9 Is this possibly a bug in the C++20 Modules implementation?

[mordante]

There seem to be two issues

• c108653 moved operator| to the ranges namespace but didn't export the named declaration
• clang-tidy no longer runs in the CI so the error was not detected

I'll look at both issues.