[Clang] std::ranges::copy in libstdc++ not constant expression.

[SainoNamkho]

https://godbolt.org/z/j3KnMGocn

#include <array>
#include <ranges>
#include <algorithm>
#include <print>

constexpr auto arr = [] {
    std::array<int, 2> arr;
    std::ranges::copy(std::array{std::array{1}, std::array{2}} | std::views::join, arr.data());
    return arr;
}();

int main()
{
    for(auto i : arr)
        std::println("{}",i);
}

Clang together with libstdc++ reports

<source>:6:16: error: constexpr variable 'arr' must be initialized by a constant expression
    5 | constexpr auto arr = [] {
      |                ^     ~~~~
    6 |     std::array<int, 2> arr;
      |     ~~~~~~~~~~~~~~~~~~~~~~~
    7 |     std::ranges::copy(std::array{std::array{1}, std::array{2}} | std::views::join, arr.data());
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    8 |     return arr;
      |     ~~~~~~~~~~~
    9 | }();
      | ~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/ranges_algobase.h:291:12: note: subobject '_M_value' is not initialized
  291 |           return {std::move(__first), std::move(__result)};
      |                   ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/ranges_algobase.h:291:12: note: in call to '_Iterator(__first)'
  291 |           return {std::move(__first), std::move(__result)};
      |                   ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/optional:230:8: note: subobject declared here
  230 |           _Up _M_value;
      |               ^

I can't see why _M_value is not initialized, the implementation of ranges library from libstdc++ is almost the same as that from libc++, so the difference might be from std::optional.

It is only rejected since clang 17: https://godbolt.org/z/354TGGrGq
@YexuanXiao

[shafik]

Note works fine w/ libc++: https://godbolt.org/z/7fe1PdjWW

[frederick-vs-ja]

This correctly compiles (Godbolt link).

#include <array>
#include <ranges>
// #include <algorithm>

constexpr auto arr = [] {
    std::array<int, 2> arr{};
    for (auto it = arr.data();
         decltype(auto) x : std::array{std::array{1}, std::array{2}} | std::views::join) {
        *it = static_cast<decltype(x)&&>(x);
        ++it;
    }
    // std::ranges::copy(arrarr | std::views::join, arr.data());
    return arr;
}();

static_assert(arr == std::array<int, 2>{1, 2});

Note that libstdc++'s join_view's iterator holds an optional. The optional in the iterator doesn't contain a value when being moved in the posted operations.

Reduced example (Godbolt link):

#include <optional>
#include <utility>

constexpr int zero = []
{
    std::optional<int> opt{};
    opt.emplace(42);
    opt.reset();
    auto opt2 [[maybe_unused]] = std::move(opt);
    return 0;
}();

[frederick-vs-ja]

libstdc++'s optional::reset unconditionally destroys the contained value (subobject), while libc++'s only does so when the optional is not trivially destructible.

https://github.com/gcc-mirror/gcc/blob/e656656e711949ef42a7e284f7cf81ca56f37374/libstdc%2B%2B-v3/include/std/optional#L280-L285

llvm-project/libcxx/include/optional

Lines 309 to 314 in 83e61d0

	_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void reset() noexcept {
	if (__engaged_) {
	__val_.~value_type();
	__engaged_ = false;
	}
	}

llvm-project/libcxx/include/optional

Lines 340 to 344 in 83e61d0

	_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void reset() noexcept {
	if (__engaged_) {
	__engaged_ = false;
	}
	}

I think I've reached a library-free example Godbolt link.

constexpr int zero = []
{
    using I = int;
    struct OptionalLike {
        union U {
            I n;
        };
        U u;
    };

    OptionalLike opt;
    opt.u.n = 42;
    opt.u.n.~I();

    auto opt2 [[maybe_unused]] = opt;
    return 0;
}();

GCC & MSVC accept this, EDG & Clang reject this with the same reason. (ICC is probably buggy here and rejects this with an unreasonable reason.)

IIUC this issue is probably covered by CWG2658.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (SainoNamkho)

https://godbolt.org/z/j3KnMGocn ```c++ #include <array> #include <ranges> #include <algorithm> #include <print>

constexpr auto arr = [] {
std::array<int, 2> arr;
std::ranges::copy(std::array{std::array{1}, std::array{2}} | std::views::join, arr.data());
return arr;
}();

int main()
{
for(auto i : arr)
std::println("{}",i);
}

Clang together with libstdc++ reports

<source>:6:16: error: constexpr variable 'arr' must be initialized by a constant expression
5 | constexpr auto arr = [] {
| ^ ~~~~
6 | std::array<int, 2> arr;
| ~~~~~~~~~~~~~~~~~~~~~~~
7 | std::ranges::copy(std::array{std::array{1}, std::array{2}} | std::views::join, arr.data());
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8 | return arr;
| ~~~~~~~~~~~
9 | }();
| ~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/ranges_algobase.h:291:12: note: subobject '_M_value' is not initialized
291 | return {std::move(__first), std::move(__result)};
| ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/bits/ranges_algobase.h:291:12: note: in call to '_Iterator(__first)'
291 | return {std::move(__first), std::move(__result)};
| ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/15.0.0/../../../../include/c++/15.0.0/optional:230:8: note: subobject declared here
230 | _Up _M_value;
| ^

I can't see why `_M_value` is not initialized, the implementation of ranges library from libstdc++ is almost the same as that from libc++, so the difference might be from `std::optional`.

It is only rejected since clang 17: https://godbolt.org/z/354TGGrGq
@<!-- -->YexuanXiao
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