Fail if a GlobalRef is being assigned to a Dual number.

[YingboMa]

[codecov-io]

Codecov Report

Merging #39 into master will increase coverage by 0.49%. The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
+ Coverage   73.26%   73.75%   +0.49%     
==========================================
  Files           7        7              
  Lines         273      282       +9     
==========================================
+ Hits          200      208       +8     
- Misses         73       74       +1

Impacted Files	Coverage Δ
src/custom_dispatch.jl	94.28% <80%> (-5.72%)	:arrow_down:
src/dual_context.jl	90.56% <0%> (+1.88%)	:arrow_up:

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[MikeInnes]

Note that this won't solve all leaks, e.g. if you have global ref = Real[0] and put a number in there while differentiating you'll get the same issue.

I could probably also get around this by, for example, doing global foo = [x::Dual] or global foo = SomeStruct(x::Dual) (and same for setfield issues).

[YingboMa]

I could probably also get around this by, for example, doing global foo = [x::Dual] or global foo = SomeStruct(x::Dual) (and same for setfield issues).

If the user wants to get a Dual, then it is the intended behavior.