pirate errors for two mistakes (#1976)

Author: mcabbott

src/Flux.jl

@@ -7,12 +7,15 @@ using MacroTools: @forward
 
 @reexport using NNlib
 using MLUtils
-import Optimisers: trainable, destructure  # before v0.13, Flux owned these functions
+import Optimisers: Optimisers, trainable, destructure  # before v0.13, Flux owned these functions
 
 using Zygote, ChainRulesCore
 using Zygote: Params, @adjoint, gradient, pullback, @nograd
 export gradient
 
+# Pirate error to catch a common mistake. (Internal function `base` because overloading `update!` is more likely to give ambiguities.)
+Optimisers.base(dx::Zygote.Grads) = error("Optimisers.jl cannot be used with Zygote.jl's implicit gradients, `Params` & `Grads`")
+
 export Chain, Dense, Maxout, SkipConnection, Parallel,
        RNN, LSTM, GRU, GRUv3,
        SamePad, Conv, CrossCor, ConvTranspose, DepthwiseConv,
@@ -38,6 +41,9 @@ include("utils.jl")
 include("onehot.jl")
 include("functor.jl")
 
+# Pirate error to catch a common mistake.
+Functors.functor(::Type{<:MLUtils.DataLoader}, x) = error("`DataLoader` does not support Functors.jl, thus functions like `Flux.gpu` will not act on its contents.")
+
 include("layers/stateless.jl")
 include("layers/basic.jl")
 include("layers/conv.jl")