Merge #1616

1616: Warn on reconstruct length mismatch r=CarloLucibello a=ToucheSir

Ref. #1601. This is kept as a plain warning for backwards compat, but perhaps we want to consider it a bugfix and error/depwarn instead?

### PR Checklist

- [x] Tests are added
- [ ] Entry in NEWS.md
- [ ] Documentation, if applicable
- [ ] API changes require approval from a committer (different from the author, if applicable)


Co-authored-by: Brian Chen <ToucheSir@users.noreply.github.com>

Author: bors[bot]

src/utils.jl

@@ -610,16 +610,24 @@ end
 
 function _restructure(m, xs)
   i = 0
-  fmap(m) do x
+  m̄ = fmap(m) do x
     x isa AbstractArray || return x
     x = reshape(xs[i.+(1:length(x))], size(x))
     i += length(x)
     return x
   end
+  length(xs) == i || @warn "Expected $(i) params, got $(length(xs))"
+  return m̄
 end
 
 @adjoint function _restructure(m, xs)
-  _restructure(m, xs), dm -> (nothing,destructure(dm)[1])
+  m̄, numel = _restructure(m, xs), length(xs)
+  function _restructure_pullback(dm)
+    xs′ = destructure(dm)[1]
+    numel == length(xs′) || @warn "Expected $(numel) params, got $(length(xs′))"
+    return (nothing, xs′)
+  end
+  return m̄, _restructure_pullback
 end
 
 """

test/utils.jl

@@ -378,6 +378,15 @@ end
       p, re = destructure(m)
       testdense(re(p), bt)
     end
+
+    @testset "restructure in gradient" begin
+      x = rand(Float32, 3, 1)
+      m = dm(zeros)
+      ∇m = gradient(m -> sum(m(x)), m)[1]
+      p, re = destructure(m)
+      ∇p = gradient(θ -> sum(re(θ)(x)), p)[1]
+      @test ∇p ≈ destructure(∇m)[1]
+    end
   end
 end