project ZeroTangent to natural tangent for some number types

Author: ToucheSir

src/projection.jl

@@ -191,6 +191,7 @@ end
 # understands, including a mix of Zeros & reals. Other cases, we just let through:
 (project::ProjectTo{<:Number})(dx::Tangent{<:Complex}) = project(Complex(dx.re, dx.im))
 (::ProjectTo{<:Number})(dx::Tangent{<:Number}) = dx
+(::ProjectTo{T})(::ZeroTangent) where T <: Real = zero(T)
 
 # Arrays
 # If we don't have a more specialized `ProjectTo` rule, we just assume that there is

test/projection.jl

@@ -37,10 +37,10 @@ struct NoSuperType end
         @test ProjectTo(1.0)(2) === 2.0
 
         # Tangents
-        ProjectTo(1.0f0 + 2im)(Tangent{ComplexF64}(; re=1, im=NoTangent())) ===
-        1.0f0 + 0.0f0im
-
-        @test 1.0 === ProjectTo(1.0)(Tangent{ComplexF64}(; re=1, im=NoTangent()))
+        complex_tangent = Tangent{ComplexF64}(; re=1, im=NoTangent())
+        @test ProjectTo(1.0f0 + 2im)(complex_tangent) === 1.0f0 + 0.0f0im
+        @test ProjectTo(1.0)(complex_tangent) === 1.0
+        @test ProjectTo(1.0)(ZeroTangent()) === 0.0
     end
 
     @testset "Dual" begin # some weird Real subtype that we should basically leave alone