row vector non-diff shortcuts

Author: mcabbott

src/projection.jl

@@ -270,6 +270,7 @@ end
 # Row vectors
 function ProjectTo(x::LinearAlgebra.AdjointAbsVec)
     sub = ProjectTo(parent(x))
+    sub isa ProjectTo{<:AbstractZero} && return sub
     return ProjectTo{Adjoint}(; parent=sub)
 end
 # Note that while [1 2; 3 4]' isa Adjoint, we use ProjectTo{Adjoint} only to encode AdjointAbsVec.
@@ -288,6 +289,7 @@ end
 
 function ProjectTo(x::LinearAlgebra.TransposeAbsVec)
     sub = ProjectTo(parent(x))
+    sub isa ProjectTo{<:AbstractZero} && return sub
     return ProjectTo{Transpose}(; parent=sub)
 end
 function (project::ProjectTo{Transpose})(dx::LinearAlgebra.AdjOrTransAbsVec)

test/projection.jl

@@ -125,7 +125,7 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
         @test_throws DimensionMismatch prefvec(Ref{Any}(1:5))
 
         @test ProjectTo(Ref(true)) isa ProjectTo{NoTangent}
-        @test_broken ProjectTo(Ref([false]')) isa ProjectTo{NoTangent}
+        @test ProjectTo(Ref([false]')) isa ProjectTo{NoTangent}
     end
 
     #####
@@ -172,6 +172,9 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
 
         # issue #410
         @test padj([NoTangent() NoTangent() NoTangent()]) === NoTangent()
+
+        @test ProjectTo(adj([true, false]))([1 2]) isa AbstractZero
+        @test ProjectTo(adj([[true], [false]])) isa ProjectTo{<:AbstractZero}
     end
 
     @testset "LinearAlgebra: dense structured matrices" begin