stop accepting Ref as a tangent for Ref

Author: mcabbott

src/projection.jl

@@ -248,8 +248,11 @@ function _projection_mismatch(axes_x::Tuple, size_dx::Tuple)
     )
 end
 
+#####
+##### `Base`, part II: return of the Tangent
+#####
+
 # Ref
-# This can't be its own tangent, so it standardises on a Tangent{<:Ref}
 function ProjectTo(x::Ref)
     sub = ProjectTo(x[])  # should we worry about isdefined(Ref{Vector{Int}}(), :x)? 
     if sub isa ProjectTo{<:AbstractZero}
@@ -258,7 +261,6 @@ function ProjectTo(x::Ref)
         return ProjectTo{Ref}(; type=typeof(x), x=sub)
     end
 end
-(project::ProjectTo{Ref})(dx::Ref) = Tangent{project.type}(; x=project.x(dx[]))
 (project::ProjectTo{Ref})(dx::Tangent) = Tangent{project.type}(; x=project.x(dx.x))
 # Since this works like a zero-array in broadcasting, it should also accept a number:
 (project::ProjectTo{Ref})(dx::Number) = Tangent{project.type}(; x=project.x(dx))

test/projection.jl

@@ -114,14 +114,14 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
 
     @testset "Base: Ref" begin
         pref = ProjectTo(Ref(2.0))
-        @test pref(Ref(3 + im)).x === 3.0
+        @test_skip pref(Ref(3 + im)).x === 3.0
         @test pref(4).x === 4.0  # also re-wraps scalars
-        @test pref(Ref{Any}(5.0)) isa Tangent{<:Base.RefValue}
+        @test_skip pref(Ref{Any}(5.0)) isa Tangent{<:Base.RefValue}
         pref2 = ProjectTo(Ref{Any}(6 + 7im))
-        @test pref2(Ref(8)).x === 8.0 + 0.0im
+        @test_skip pref2(Ref(8)).x === 8.0 + 0.0im
 
         prefvec = ProjectTo(Ref([1, 2, 3 + 4im]))  # recurses into contents
-        @test prefvec(Ref(1:3)).x isa Vector{ComplexF64}
+        @test_skip prefvec(Ref(1:3)).x isa Vector{ComplexF64}
         @test_throws DimensionMismatch prefvec(Ref{Any}(1:5))
 
         @test ProjectTo(Ref(true)) isa ProjectTo{NoTangent}