address review comments

test/compiler.jl

@@ -162,6 +162,7 @@ end
     g = Gaussian(randn(3), randn(3, 3))
     y, back = @inferred pullback(x -> x.m, g)
     @test y == getfield(g, :m)
+    # This type instability is due to the handling of non-bitstypes in `accum_param`
     @test Base.return_types(back, Tuple{Vector{Float64}}) == Any[Union{Tuple{Nothing}, typeof(((m = [1.0, 0.0, 0.0], P = nothing),))}]
     @test back([1., 0, 0]) == ((m = [1.0, 0.0, 0.0], P = nothing),)
 
@@ -175,10 +176,10 @@ end
     g = Gaussian(randn(3), randn(3, 3))
     y, back = @inferred pullback(x -> x.m, g)
 
-    Zygote._pullback(::typeof(getproperty), g::Gaussian, s::Symbol) = 3getfield(g, s), Δ -> (nothing, (; ((:m, :P) .=> nothing)..., s => 3Δ), nothing)
+    Zygote._pullback(::Zygote.AContext, ::typeof(getproperty), g::Gaussian, s::Symbol) = 3getfield(g, s), Δ -> (nothing, (; ((:m, :P) .=> nothing)..., s => 3Δ), nothing)
     y, back = pullback(x -> x.m, g)
-    @test_broken y == 3getfield(g, :m)
-    @test_broken back([1., 0, 0]) == ((m = [3.0, 0.0, 0.0], P = nothing),)
+    @test y == 3getfield(g, :m)
+    @test back([1., 0, 0]) == ((m = [3.0, 0.0, 0.0], P = nothing),)
 
 
     Gaussian = _Gaussian(:rrule)
@@ -189,6 +190,12 @@ end
     y, back = pullback(x -> x.m, g)
     @test y == 4getfield(g, :m)
     @test back([1., 0, 0]) == ((m = [4.0, 0.0, 0.0], P = nothing),)
+
+    Gaussian = _Gaussian(:bitstype)
+    g = Gaussian(randn(), randn())
+    y, back = @inferred pullback(x -> x.m, g)
+    @test y == getfield(g, :m)
+    @test @inferred(back(1.0)) == ((m = 1.0, P = nothing),)
 end
 
 # issue 897