Accumulate mutable structs with Ref

src/lib/lib.jl

@@ -28,6 +28,16 @@ function accum(x::RefValue, y::RefValue)
   @assert x === y
   return x
 end
+function accum(x::NamedTuple, ref::RefValue)
+    # We do not actually do any accumulation here, because the ref will already have been mutated.
+  fieldnames(typeof(ref[])) ⊆ fieldnames(typeof(x)) || throw(ArgumentError("$(ref[]) keys from Ref must be a subset of $x keys"))
+  ref
+end
+function accum(ref::RefValue, x::NamedTuple)
+    # We do not actually do any accumulation here, because the ref will already have been mutated.
+  fieldnames(typeof(x)) ⊆ fieldnames(typeof(ref[])) || throw(ArgumentError("$x keys must be a subset of $(ref[]) keys from Ref"))
+  ref
+end
 
 accum(x::NamedTuple, y::ChainRulesCore.Tangent) = accum(x, wrap_chainrules_output(y))
 accum(x::ChainRulesCore.Tangent, y::NamedTuple) = accum(wrap_chainrules_output(x), y)

test/features_tests.jl

@@ -480,6 +480,19 @@ let
   @test back(1.) == ((1.0,),)
 end
 
+mutable struct MWEGetter{G, U}
+  idxs::G
+  u::U
+end
+
+u = ones(3)
+idxs = [1, 2]
+mwe1574 = MWEGetter(idxs, u)
+
+function fn1574(mwe)
+  map(i -> mwe.u[i], mwe.idxs)
+end
+
 @testset "mutable struct, including Ref" begin
   # Zygote's representation is Base.RefValue{Any}((value = 7.0,)), but the
   # map to ChainRules types and back normalises to (value = 7.0,) same as struct:
@@ -506,6 +519,9 @@ end
   # Broadcasting over Ref is handled specially. Tested elsewhere too.
   @test gradient(x -> sum(sum, x .* [1,2,3]), Ref([4,5])) == ((x = [6.0, 6.0],),)
   @test gradient(x -> sum(sum, Ref(x) .* [1,2,3]), [4,5]) == ([6.0, 6.0],)
+
+  # Broadcasting/ Mapping over Mutables with differring graphs for fields
+  @test gradient(x -> sum(fn1574(x)), mwe1574)[1] == (; idxs = [nothing, nothing], u = [1.0, 1.0, 0.0])
 end
 
 @testset "mutable accum_param bugs" begin