Needed in https://github.com/JuliaDiff/ChainRulesTestUtils.jl/pull/224 (see [comment](https://github.com/JuliaDiff/ChainRulesTestUtils.jl/pull/224#issuecomment-964505798)) [Simplest possible implementation](https://github.com/JuliaDiff/ChainRulesCore.jl/pull/488#issuecomment-964616104).
