[SOLVED] Error with C++20 ranges and std::views::take

Issue

I was experimenting with C++20 ranges and I got the following strange behavior when compiling with GCC 11.1.0 and CMake 3.20.3. Specifically, the following code doesn't compile:

auto Foo() {
    std::vector<long int> x{1, 2, 3, 4, 5, 6};
    return std::views::all(x) | std::views::take(x.size());
    // return std::views::all(x) | std::views::take(static_cast<int>(x.size()));
}

resulting with very long error messages such as

[...] error: no match for ‘operator|’ (operand types are ‘std::ranges::ref_view<std::vector<long int> >’ and ‘std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>’)
[build]   230 |     return std::views::all(x) | std::views::take(x.size());
[build]       |            ~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
[build]       |                           |                     |
[build]       |                           |                     std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>
[build]       |                           std::ranges::ref_view<std::vector<long int> >

and

[build] /usr/include/c++/11/ranges:739:13:   required for the satisfaction of ‘__adaptor_invocable<_Self, _Range>’ [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>; _Range = std::ranges::ref_view<std::vector<long int, std::allocator<long int> > >]
[build] /usr/include/c++/11/ranges:740:9:   in requirements  [with _Args = {std::ranges::ref_view<std::vector<long int, std::allocator<long int> > >}; _Adaptor = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>]
[build] /usr/include/c++/11/ranges:740:44: note: the required expression ‘declval<_Adaptor>()((declval<_Args>)()...)’ is invalid, because
[build]   740 |       = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
[build]       |                    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
[build] /usr/include/c++/11/ranges:740:44: error: use of deleted function ‘constexpr auto std::ranges::views::__adaptor::_Partial<_Adaptor, _Arg>::operator()(_Range&&) const && [with _Range = std::ranges::ref_view<std::vector<long int> >; _Adaptor = std::ranges::views::_Take; _Arg = long unsigned int]’
[build] /usr/include/c++/11/ranges:862:9: note: declared here
[build]   862 |         operator()(_Range&& __r) const && = delete;
[build]       |         ^~~~~~~~
[build] In file included from /usr/include/c++/11/streambuf:41,
[build]                  from /usr/include/c++/11/bits/streambuf_iterator.h:35,
[build]                  from /usr/include/c++/11/iterator:66,
[build]                  from /usr/include/c++/11/ranges:43,
[build]                  from ../mmpc/test/parametric_controller.test.cpp:27:
[build] /usr/include/c++/11/bits/ios_base.h:87:3: note: candidate: ‘constexpr std::_Ios_Fmtflags std::operator|(std::_Ios_Fmtflags, std::_Ios_Fmtflags)’
[build]    87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:87:27: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Fmtflags’
[build]    87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
[build]       |             ~~~~~~~~~~~~~~^~~
[build] /usr/include/c++/11/bits/ios_base.h:129:3: note: candidate: ‘constexpr std::_Ios_Openmode std::operator|(std::_Ios_Openmode, std::_Ios_Openmode)’
[build]   129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:129:27: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Openmode’
[build]   129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
[build]       |             ~~~~~~~~~~~~~~^~~
[build] /usr/include/c++/11/bits/ios_base.h:169:3: note: candidate: ‘constexpr std::_Ios_Iostate std::operator|(std::_Ios_Iostate, std::_Ios_Iostate)’
[build]   169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:169:26: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Iostate’
[build]   169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
[build]       |             ~~~~~~~~~~~~~^~~

However, when I convert the size of x to long int as in the commented line everything compiles fine. Is this behavior expected? Or is something wrong with my compiler setup?

Eventually, when I use Clang 12.0.1 nothing compiles at all and I get the following error:

[build] /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/ranges:3392:19: error: missing 'typename' prior to dependent type name 'iterator_traits<iterator_t<_Base>>::iterator_category'
[build]             using _Cat = iterator_traits<iterator_t<_Base>>::iterator_category;
[build]                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Shouldn't Clang 12.0.1 support the features of C++20 including ranges? Why do I get this error?

Solution

This is a library bug that will be fixed by P2367, since this is specified to do take_view{E, F} (which would be a narrowing conversion) instead of just take_view(E, F) (which would be fine). In the meantime, you have to do the cast accordingly.

---

Also this:

std::views::all(x) | std::views::take(x.size());

is just a long way of writing:

x | std::views::take(x.size());

You should rarely (if ever?) need to write views::all in user code. The library does this for you.

---

Shouldn't Clang 12.0.1 support the features of C++20 including ranges? Why do I get this error?

This has nothing to do with ranges, clang just doesn't fully support the C++20 language features that libstdc++ uses in its implementation (in this case Down with typename!).

Answered By - Barry