[devmotion]

Tangent defines no Broadcast.broadcastable, so broadcasting over one falls back to collect, which iterates over the backing and produces a Vector. That is wrong in different ways depending on the backing:

julia> Tangent{Any}(1.0, [2.0]) .* 2          # tuple: element types are lost
2-element Vector{Any}:
 2.0
  [4.0]

julia> Tangent{Foo}(; y=2.0, x=1.0) .* 2      # struct: broadcasts over the fields, in keyword order
2-element Vector{Float64}:
 4.0
 2.0

julia> Tangent{Dict}(Dict(:a => 1.0)) .* 2     # Dict: broadcasts over the `Pair`s
ERROR: MethodError: no method matching *(::Pair{Symbol, Float64}, ::Int64)

This PR makes broadcasting over a StructuralTangent (Tangent and MutableTangent) throw an ArgumentError, like Base does for NamedTuples and Dicts:

julia> Tangent{Any}(1.0, 2.0) .* 2
ERROR: ArgumentError: broadcasting over structural tangents is not supported; use `map` or arithmetic

Why not broadcast like the primal?

An earlier version of this PR returned the backing for tuple-backed tangents and Ref(tangent) otherwise. No such rule is correct in general, because the shape a tangent should broadcast with depends on the broadcast, not on the tangent:

• For Tangent{Any} the primal is unknown, and the constructor accepts any backing.
• A tuple tangent is a container in a tuple broadcast, but a single element in a 0-dim broadcast with a tuple-valued output.
• A structural tangent of an array type (e.g. Tangent{SVector}(; data=...)) belongs to a container. Treating it as a scalar made ChainRules' broadcast pullback for x -> x^3 silently return [NoTangent(), NoTangent()].

Only the code doing the broadcast knows which shape is meant. For example, ChainRules' broadcast rules know the output ys, so they should unwrap the cotangent there (as _no_tuple_tangent already does in mapreduce.jl).

Compatibility

Broadcasting over a structural tangent used to return a (mostly wrong) Vector; it now throws. In particular, ChainRules' generic broadcast pullbacks currently rely on this fallback when they get a Tangent{Any,<:Tuple} cotangent from Zygote for tuple broadcasts. They need to unwrap it before this is released. The version is therefore bumped to 1.27.0.

The unrelated fix for tuple projectors that was part of this PR is now in #725.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.56%. Comparing base (4e9c8aa) to head (755baf2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #724      +/-   ##
==========================================
+ Coverage   93.54%   93.56%   +0.01%     
==========================================
  Files          15       15              
  Lines        1022     1025       +3     
==========================================
+ Hits          956      959       +3     
  Misses         66       66              

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