Add ProjectTo(::Any) = identity

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.4.0"
+version = "1.5.0"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/projection.jl

@@ -84,7 +84,8 @@ Returns a `ProjectTo{T}` functor which projects a differential `dx` onto the
 relevant tangent space for `x`.
 
 At present this undersands only `x::Number`, `x::AbstractArray` and `x::Ref`.
-It should not be called on arguments of an `rrule` method which accepts other types.
+Called on unknown types it will now simply return `identity`, thus can be safely
+applied to arbitrary `rrule` arguments.
 
 # Examples
 ```jldoctest
@@ -112,7 +113,7 @@ julia> ProjectTo([1 2; 3 4]')  # no special structure, integers are promoted to
 ProjectTo{AbstractArray}(element = ProjectTo{Float64}(), axes = (Base.OneTo(2), Base.OneTo(2)))
 ```
 """
-ProjectTo(::Any) # just to attach docstring
+ProjectTo(::Any) = identity
 
 # Generic
 (::ProjectTo{T})(dx::AbstractZero) where {T} = dx
@@ -143,6 +144,11 @@ ProjectTo{P}(::NamedTuple{T, <:Tuple{_PZ, Vararg{<:_PZ}}}) where {P,T} = Project
 # Bool
 ProjectTo(::Bool) = ProjectTo{NoTangent}()  # same projector as ProjectTo(::AbstractZero) above
 
+# Other never-differentiable types
+for T in [:Symbol, :Char, :AbstractString, :RoundingMode, :IndexStyle]
+    @eval ProjectTo(::$T) = ProjectTo{NoTangent}()
+end
+
 # Numbers
 ProjectTo(::Real) = ProjectTo{Real}()
 ProjectTo(::Complex) = ProjectTo{Complex}()

test/projection.jl

@@ -11,6 +11,9 @@ Base.real(x::Dual) = x
 Base.float(x::Dual) = Dual(float(x.value), float(x.partial))
 Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
 
+# Trivial struct
+struct NoSuperType end
+
 @testset "projection" begin
 
     #####
@@ -24,7 +27,6 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
         @test ProjectTo(2.0+3.0im)(1+1im) === 1.0+1.0im
         @test ProjectTo(2.0)(1+1im) === 1.0
 
-
         # storage
         @test ProjectTo(1)(pi) === pi
         @test ProjectTo(1 + im)(pi) === ComplexF64(pi)
@@ -94,9 +96,10 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
         @test y1[1] == [1 2]
         @test !(y1 isa Adjoint) && !(y1[1] isa Adjoint)
 
-        # arrays of unknown things
-        @test_throws MethodError ProjectTo([:x, :y])
-        @test_throws MethodError ProjectTo(Any[:x, :y])
+        # arrays of other things
+        @test ProjectTo([:x, :y]) isa ProjectTo{NoTangent}
+        @test ProjectTo(Any['x', "y"]) isa ProjectTo{NoTangent}
+        @test ProjectTo([(1,2), (3,4), (5,6)]) isa ProjectTo{AbstractArray}
 
         @test ProjectTo(Any[1, 2])(1:2) == [1.0, 2.0]  # projects each number.
         @test Tuple(ProjectTo(Any[1, 2 + 3im])(1:2)) === (1.0, 2.0 + 0.0im)
@@ -140,6 +143,12 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
         @test ProjectTo(Ref([false]')) isa ProjectTo{NoTangent}
     end
 
+    @testset "Base: non-diff" begin
+        @test ProjectTo(:a)(1) == NoTangent()
+        @test ProjectTo('b')(2) == NoTangent()
+        @test ProjectTo("cde")(345) == NoTangent()
+    end
+
     #####
     ##### `LinearAlgebra`
     #####
@@ -301,6 +310,10 @@ Base.zero(x::Dual) = Dual(zero(x.value), zero(x.partial))
     ##### `ChainRulesCore`
     #####
 
+    @testset "pass-through" begin
+        @test ProjectTo(NoSuperType()) === identity
+    end
+
     @testset "AbstractZero" begin
         pz = ProjectTo(ZeroTangent())
         pz(0) == NoTangent()