Reshaping a row major matrix by row major returns a vector whose iterator isn't `std::contiguous_iterator`.

Summary

Reshaping a row major matrix by row major returns a column major vector.

Environment

• Operating System : Linux
• Architecture : x64
• Eigen Version : 3.3.9
• Compiler Version : Gcc13.2.1
• Compile Flags : -O2 -march=x86-64-v3
• Vector Extension : AVX2

Minimal Example

#include <span>

#include "Eigen/Dense"

int main() {
    Eigen::Matrix3f m;
    std::span<const float, 9>(m.reshaped<Eigen::StorageOptions::RowMajor>().begin(), 9);
}

godbolt

Steps to reproduce

1. Open the above godbolt link.

What is the current bug behavior?

When I reshape a row major matrix by row major, the iterator category of the returned vector is random access even though it can be contiguous.

What is the expected correct behavior?

When I reshape a row major matrix by row major, the iterator category of the returned vector should be contiguous.

Anything else that might help

I think one of the causes is that the reshaped member function always returns a column vector. Actually, I tried making reshaped return a row vector, then no compile error appeared.

• Have a plan to fix this issue.