implement projection for InplaceableThunk (#541)

Author: mzgubic

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.12"
+version = "1.12.1"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/projection.jl

@@ -122,6 +122,7 @@ ProjectTo(::Any) = identity
 
 # Thunks
 (project::ProjectTo)(dx::Thunk) = Thunk(project ∘ dx.f)
+(project::ProjectTo)(dx::InplaceableThunk) = project(dx.val)
 
 # Zero
 ProjectTo(::AbstractZero) = ProjectTo{NoTangent}()  # Any x::Zero in forward pass makes this one projector,

test/projection.jl

@@ -427,6 +427,13 @@ struct NoSuperType end
         @test unthunk(pth) === 6.0 + 0.0im
     end
 
+    @testset "InplaceableThunk" begin
+        it = InplaceableThunk(x -> x + 6, @thunk 1 + 2 + 3)
+        pt = ProjectTo(4 + 5im)(it)
+        @test pt isa Thunk
+        @test unthunk(pt) === 6.0 + 0.0im
+    end
+
     @testset "Tangent" begin
         x = 1:3.0
         dx = Tangent{typeof(x)}(; step=0.1, ref=NoTangent())