Fix compile issue with clang 19.1.1

[starboerg]

Checklist

• [x ] The title and commit message(s) are descriptive.
• [ x] Small commits made to fix your PR have been squashed to avoid history pollution.
• Tests have been added for new features or bug fixes.
• API of new functions and classes are documented.

Description

Parallel assigner branches had a probably outdated use of the simd stepper template.