[primitives] Spell ternary operator compatibly with new AutoDiff#17533
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The new AutoDiff will have a breaking change: its derivatives() mutable accessor will no longer guide the conditional ternary operator to construct a temporary VectorXd. There is no longer a "common type" between a constant-expression eigen xpr vector and an AutoDiff derivatives vector (both a will be eigen xprs, of different types). We plan to add sugar to make this easier down the road, but in any case forcing the constant vector to materialize in test code is not any great loss, and it's easier for the commit train to land this now and get it out of the way.
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Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status:complete! all discussions resolved, LGTM from assignee RussTedrake(platform) (waiting on @jwnimmer-tri)
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The new AutoDiff will have a breaking change: its
derivatives()mutable accessor will no longer guide the conditional ternary operator to construct a temporaryEigen::VectorXd. There is no longer a "common type" between a constant-expression eigen xpr vector and a mutable AutoDiff derivatives vector (both a will be eigen xprs, of different types).We plan to add sugar to make this easier down the road (i.e., without a ternary operator), but in any case forcing the constant vector to materialize in test code is not any great loss, and it's easier for the commit train to land this now and get it out of the way.
(We can't merge the sugar until we merge the new
derivatives()first, so it's a bit of a Catch-22.)Towards #17492.
This change is