[SAY-5]

Fixes #314.

ifelse(tracer_cond, c1, c2) where both branches are plain constants (not tracers) hit output_union with no matching method and threw MethodError: no method matching output_union(::Float64, ::Float64). The existing scalar fallbacks only cover the cases where at least one branch is an AbstractTracer.

Added a fallback for when neither branch is a tracer. In that case the result carries no dependency on the inputs, so returning either constant gives the correct (empty) sparsity pattern. Also added a gradient test for the both-constant case next to the existing single-constant ones.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (3dc3b2f) to head (fdcc6e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #325   +/-   ##
=======================================
  Coverage   93.81%   93.82%           
=======================================
  Files          28       28           
  Lines        1100     1101    +1     
=======================================
+ Hits         1032     1033    +1     
  Misses         68       68           

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[adrhill]

Maybe we should add a size assertion on both inputs while we are at it.

[SAY-5]

The scalar Base.ifelse(::AbstractTracer, x, y) entry already asserts size(x) != size(y) before dispatching to output_union, so mismatched branch sizes are caught there. The gap is the array output_union overloads (the ones under the # TODO: add test marker), which assume matching sizes without checking. Happy to add a matching size(tx) == size(ty) assertion to those two-array methods and drop the TODO by covering them with a test. Want me to go that route, or did you have a different pair of inputs in mind?

[adrhill]

Thanks! :)