Treat pairs(NamedTuple) as NamedTuple for indexing

src/lib/base.jl

@@ -119,11 +119,11 @@ end
 
 # named tuple
 @adjoint function pairs(t::NamedTuple{N}) where N
-  
+
   pairs_namedtuple_pullback(dx::NamedTuple) = (dx.data,)
 
   pairs_namedtuple_pullback(dx::Tuple{}) = (NamedTuple(),)
-  
+
   function pairs_namedtuple_pullback(Δ::Dict)
     t0 = map(zero, t)
     for (idx, v) in Δ
@@ -145,6 +145,30 @@ else
   @adjoint merge(nt::NamedTuple, dict::Dict) = pullback(merge, nt, (;dict...))
 end
 
+# Keyword arguments pretend to be a Dict, but are secretly wrapping a NamedTuple.
+# We can treat them much the same, just with some plumbing to handle the extra `itr` field.
+function _pullback(::AContext, ::typeof(getindex),
+                   ps::Iterators.Pairs{<:Any,<:Any,<:Any,<:NamedTuple}, k)
+  # So we don't close over kwarg values in the pullback
+  data = map(_ -> nothing, NamedTuple(ps))
+  function kwargs_getindex_pullback(Δ)
+    dps = (data = Base.setindex(data, Δ, k), itr = nothing)
+    return (nothing, dps, nothing)
+  end
+  return ps[k], kwargs_getindex_pullback
+end
+
+function _pullback(cx::AContext, ::typeof(literal_getindex),
+                   ps::Iterators.Pairs{<:Any,<:Any,<:Any,<:NamedTuple}, ::Val{K}) where K
+  val, gf_back = _pullback(cx, literal_getfield, NamedTuple(ps), Val(K))
+  function kwargs_literal_getindex_pullback(Δ)
+    dps = (data = gf_back(Δ)[2], itr = nothing)
+    return (nothing, dps, nothing)
+  end
+  return val, kwargs_literal_getindex_pullback
+end
+
+# Misc.
 @adjoint function Base.getfield(p::Pair, i::Int)
     function pair_getfield_pullback(Δ)
         f, s = i == 1 ? (Δ, nothing) : (nothing, Δ)

test/features.jl

@@ -552,6 +552,17 @@ end
   @test gradient(x -> x[].a, Ref((a=1, b=2))) == ((x = (a = 1, b = nothing),),)
   @test gradient(x -> x[1][].a, [Ref((a=1, b=2)), Ref((a=3, b=4))]) == ([(x = (a = 1, b = nothing),), nothing],)
   @test gradient(x -> x[1].a, [(a=1, b=2), "three"]) == ([(a = 1, b = nothing), nothing],)
+
+  @testset "indexing kwargs" begin
+    inner_lit_index(; kwargs...) = kwargs[:x]
+    outer_lit_index(; kwargs...) = inner_lit_index(; x=kwargs[:x])
+
+    inner_dyn_index(k; kwargs...) = kwargs[k]
+    outer_dyn_index(k; kwargs...) = inner_dyn_index(k; x=kwargs[k])
+
+    @test gradient(x -> outer_lit_index(; x), 0.0) == (1.0,)
+    @test gradient((x, k) -> outer_dyn_index(k; x), 0.0, :x) == (1.0, nothing)
+  end
 end
 
 function type_test()
@@ -562,7 +573,7 @@ end
 
 @testset "Pairs" begin
   @test (x->10*pairs((a=x, b=2))[1])'(100) === 10.0
-  @test (x->10*pairs((a=x, b=2))[2])'(100) === 0
+  @test (x->10*pairs((a=x, b=2))[2])'(100) === nothing
   foo(;kw...) = 1
   @test gradient(() -> foo(a=1,b=2.0)) === ()
 
@@ -578,8 +589,8 @@ end
 @testset "kwarg splatting, pass in object" begin
   g(; kwargs...) = kwargs[:x] * kwargs[:z]
   h(somedata) = g(; somedata...)
-  @test gradient(h, (; x=3.0, y=4.0, z=2.3)) == ((x = 2.3, y = 0.0, z = 3.0),)
-  @test gradient(h, Dict(:x=>3.0, :y=>4.0, :z=>2.3)) == ((y = 0.0, z = 3.0, x = 2.3),)
+  @test gradient(h, (; x=3.0, y=4.0, z=2.3)) == ((x = 2.3, y = nothing, z = 3.0),)
+  @test gradient(h, Dict(:x=>3.0, :y=>4.0, :z=>2.3)) == ((y = nothing, z = 3.0, x = 2.3),)
 end
 
 @testset "Iterators" begin