<ranges>: views::cartesian_product and views::filter emit spurious warning C4324 'structure was padded due to alignment specifier'

[StephanTLavavej]

D:\GitHub\STL\out\x64>type meow.cpp

#include <print>
#include <ranges>
#include <vector>
using namespace std;

int main() {
    const vector<int> vec{11, 22, 33};
    auto always_true = [](const auto&) { return true; };

    for (const auto& [e0, e1] : views::cartesian_product(vec, vec) | views::filter(always_true)) {
        println("{} {}", e0, e1);
    }
}

D:\GitHub\STL\out\x64>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od meow.cpp
meow.cpp
D:\GitHub\STL\out\x64\out\inc\ranges(2403): warning C4324: 'std::ranges::filter_view<std::ranges::cartesian_product_view<std::ranges::ref_view<const std::vector<int,std::allocator<int>>>,std::ranges::ref_view<const std::vector<int,std::allocator<int>>>>,main::<lambda_1>>': structure was padded due to alignment specifier
D:\GitHub\STL\out\x64\out\inc\ranges(2403): note: the template instantiation context (the oldest one first) is
D:\GitHub\STL\out\x64\out\inc\ranges(198): note: while compiling class template member function 'decltype(auto) std::ranges::_Range_closure<std::ranges::views::_Filter_fn,main::<lambda_1>>::operator ()(_Ty &&) noexcept(<expr>) &'
D:\GitHub\STL\out\x64\out\inc\ranges(930): note: while evaluating concept 'invocable<std::ranges::views::_Filter_fn,std::ranges::cartesian_product_view<std::ranges::ref_view<std::vector<int,std::allocator<int> > const >,std::ranges::ref_view<std::vector<int,std::allocator<int> > const > >,`main'::`2'::<lambda_1> &>'
D:\GitHub\STL\out\x64\out\inc\ranges(2418): note: see reference to class template instantiation 'std::ranges::filter_view<std::ranges::cartesian_product_view<std::ranges::ref_view<const std::vector<int,std::allocator<int>>>,std::ranges::ref_view<const std::vector<int,std::allocator<int>>>>,main::<lambda_1>>' being compiled

D:\GitHub\STL\out\x64>meow
11 11
11 22
11 33
22 11
22 22
22 33
33 11
33 22
33 33

[cpplearner]

Reduced:

#include <__msvc_int128.hpp>
using namespace std;

#pragma pack(push, 8)
struct cached_position { std::_Signed128 off = -1; };

struct fake_filter_view : cached_position {
    int x;
};
#pragma pack(pop)

[cpplearner]

So the warning is correct: _Signed128 does have an alignment specifier (inherited from _Base128), and filter_view does gain extra padding because of the _Signed128 member.

It seems to me that the warning is harmless and can be dismissed.