Ok, it looks like a recent change to type intersection changed this, but I'm not totally confident it won't change again, so let's just keep the method for now.

Codecov Report

Merging #52 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #52      +/-   ##
=========================================
- Coverage   74.95%   74.9%   -0.05%     
=========================================
  Files          13      13              
  Lines         519     518       -1     
=========================================
- Hits          389     388       -1     
  Misses        130     130

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I think removing the method seems to work on both julia 1.1 and 1.2, so maybe that is the right way to do it for now?

Would it be crazy to dynamically determine if you need disambiguation at precompile-time? Something like this:

try
    convert(Any, NA)
    true
catch err
    err isa MethodError || rethrow()
    false
end || begin
    Base.convert(::Type{Any}, ::DataValue{Union{}}) = NA
end

That seems very involved :) I almost feel having the static version check there is simpler?

I thought it's nice to be able to define it only using stable public API; i.e., it does/will not have to record which past/future Julia versions need this definition.

@JeffBezanson, do you have an opinion about that kind of approach?