also fix a method ambiguity

Author: mcabbott

src/projection.jl

@@ -124,7 +124,7 @@ ProjectTo(::Any) # just to attach docstring
 # Zero
 ProjectTo(::AbstractZero) = ProjectTo{NoTangent}()  # Any x::Zero in forward pass makes this one projector,
 (::ProjectTo{NoTangent})(dx) = NoTangent()          # but this is the projection only for nonzero gradients,
-(::ProjectTo{NoTangent})(::NoTangent) = NoTangent() # and this one solves an ambiguity.
+(::ProjectTo{NoTangent})(dx::AbstractZero) = dx     # and this one solves an ambiguity.
 
 # Also, any explicit construction with fields, where all fields project to zero, itself
 # projects to zero. This simplifies projectors for wrapper types like Diagonal([true, false]).

test/projection.jl

@@ -299,11 +299,12 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
     @testset "AbstractZero" begin
         pz = ProjectTo(ZeroTangent())
         pz(0) == NoTangent()
-        @test_broken pz(ZeroTangent()) === ZeroTangent()  # not sure how NB this is to preserve
+        @test pz(ZeroTangent()) === ZeroTangent()  # not sure how NB this is to preserve
         @test pz(NoTangent()) === NoTangent()
 
         pb = ProjectTo(true) # Bool is categorical
         @test pb(2) === NoTangent()
+        @test pb(ZeroTangent()) isa AbstractZero  # was a method ambiguity!
 
         # all projectors preserve Zero, and specific type, via one fallback method:
         @test ProjectTo(pi)(ZeroTangent()) === ZeroTangent()