give up on Any[NoTangent(), ...] case

Author: mcabbott

src/projection.jl

@@ -199,11 +199,7 @@ function ProjectTo(xs::AbstractArray)
 end
 
 function (project::ProjectTo{AbstractArray})(dx::AbstractArray{S,M}) where {S,M}
-    # Trivial case:
-    if all(el -> el isa AbstractZero, dx)
-        return NoTangent()
-    end
-    # Now deal with shape. The rule is that we reshape to add or remove trivial dimensions
+    # First deal with shape. The rule is that we reshape to add or remove trivial dimensions
     # like dx = ones(4,1), where x = ones(4), but throw an error on dx = ones(1,4) etc.
     dy = if axes(dx) == project.axes
         dx
@@ -226,6 +222,11 @@ function (project::ProjectTo{AbstractArray})(dx::AbstractArray{S,M}) where {S,M}
     return dz
 end
 
+# Trivial case -- won't collapse Any[NoTangent(), NoTangent()]
+function (project::ProjectTo{AbstractArray})(dx::AbstractArray{<:AbstractZero})
+    return NoTangent()
+end
+
 # Row vectors aren't acceptable as gradients for 1-row matrices:
 function (project::ProjectTo{AbstractArray})(dx::LinearAlgebra.AdjOrTransAbsVec)
     return project(reshape(vec(dx), 1, :))

test/projection.jl

@@ -72,7 +72,6 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
 
         # some bugs
         @test pvec3(fill(NoTangent(), 3)) === NoTangent() #410, was an array of such
-        @test pvec3(Any[NoTangent(), NoTangent(), NoTangent()]) === NoTangent()
         @test ProjectTo([pi])([1]) isa Vector{Int} #423, was Irrational -> Bool -> NoTangent
     end