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Don't claim arrays with a Union{} element type for TrackedStyle #325

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Merged
devmotion merged 1 commit into from
Oct 25, 2026
Merged

Don't claim arrays with a Union{} element type for TrackedStyle #325

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12 changes: 10 additions & 2 deletions src/derivatives/broadcast.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -38,8 +38,14 @@ struct TrackedStyle{N} <: AbstractArrayStyle{N} end
(::Type{<:TrackedStyle})(::Val{N}) where {N} = TrackedStyle{N}()

Broadcast.BroadcastStyle(::Type{<:TrackedReal}) = TrackedStyle{0}()
# only an element type that fixes `D`, since `getouttype` reads it off
Broadcast.BroadcastStyle(::Type{<:AbstractArray{<:TrackedReal{<:Any, D}, N}}) where {D, N} =
# invariant element types that fix `D`, so that `Union{}` (a subtype of every type) does not match
Broadcast.BroadcastStyle(::Type{<:AbstractArray{TrackedReal{V, D, O}, N}}) where {V, D, O, N} =
TrackedStyle{N}()
Broadcast.BroadcastStyle(::Type{<:AbstractArray{TrackedReal{V, D, O} where {O}, N}}) where {V, D, N} =
TrackedStyle{N}()
Broadcast.BroadcastStyle(::Type{<:AbstractArray{TrackedReal{V, D, O} where {V <: Real}, N}}) where {D, O, N} =
TrackedStyle{N}()
Broadcast.BroadcastStyle(::Type{<:AbstractArray{TrackedReal{V, D, O} where {V <: Real, O}, N}}) where {D, N} =
TrackedStyle{N}()

# `AbstractArrayStyle{Any}` carries `Any` as its dimension, which `max` cannot compare
Expand Down Expand Up @@ -101,6 +107,8 @@ end

getouttype(::TrackedReal{<:Any, D}) where {D} = D
getouttype(::AbstractArray{<:TrackedReal{<:Any, D}}) where {D} = D
# `Union{} <: TrackedReal`, but binds no `D`
getouttype(::AbstractArray{Union{}}) = Union{}
getouttype(::Any) = Union{}

# a perturbation riding on an argument ends up in the derivative, so `D` has to hold it. An
Expand Down
15 changes: 15 additions & 0 deletions test/derivatives/BroadcastTests.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,21 @@ end
# `Base` styles scalars as `AbstractArrayStyle{0}`
@test BroadcastStyle(typeof(track(rand(), tp))) === TrackedStyle{0}()
@test TrackedStyle{1}(Val(2)) === TrackedStyle{2}()
# element types that leave the value type or the origin free
@test BroadcastStyle(Vector{TrackedReal{Float64, Float64}}) === TrackedStyle{1}()
@test BroadcastStyle(Vector{TrackedReal{<:Real, Float64}}) === TrackedStyle{1}()
# `TrackedReal{<:Real, Float64, Nothing}`, as built by a comprehension over mixed value types
a = TrackedReal(1.0, 0.0)
b = TrackedReal(ForwardDiff.Dual(1.0, 1.0), 0.0)
@test BroadcastStyle(typeof([x for x in Real[a, b]])) === TrackedStyle{1}()
end

@testset "arrays that hold no element keep `Base`'s style (#324)" begin
# `Union{} <: TrackedReal`
@test BroadcastStyle(Vector{Union{}}) === DefaultArrayStyle{1}()
@test BroadcastStyle(Matrix{Union{}}) === DefaultArrayStyle{2}()
@test String.(Union{}[]) isa Vector{Union{}}
@test isempty(track(1.0, InstructionTape()) .+ Union{}[])
end

@testset "`BroadcastStyle` precedence" begin
Expand Down
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