Cases in with convert has dispatch ambiguity

[bkamins]

We have the following behaviors:

julia> convert(Union{}, 1)
ERROR: MethodError: convert(::Type{Union{}}, ::Int64) is ambiguous. Candidates:

julia> convert(Missing, 1) # the same - falls back to the previous one internally
ERROR: MethodError: convert(::Type{Union{}}, ::Int64) is ambiguous. Candidates:

@vtjnash - I am pinging you as you have last changed the relevant convert methods.

[vtjnash]

Ah, this case might need to be added explicitly to nonmissingtype_checked?

[bkamins]

This would solve convert(Missing, 1), but convert(Union{}, 1) problem would still be present (probably no one in practice this will never happen, but just for completeness of definitions in Base I think it would be good to fix it).

Also the same problem as with nonmissingtype_checked you mention is with nonnothingtype_checked:

julia> Base.nonnothingtype_checked(Nothing)
Union{}

[vtjnash]

no, that is intentionally ambiguous, since it is often unavoidable by package authors

yep, that method should be updated to be the same also

[bkamins]

The issue with convert(Missing, 1) is still present on nightly even with nonmissingtype_checked.

I think we should add:

convert(::Type{Missing}, ::Missing) = missing
convert(::Type{Missing}, ::T) where {T} = throw(ArgumentError("cannot convert value of type $T to `Missing`"))

Of course custom types could override this default conversion rule.

[vtjnash]

Yes, this issue is still open, and my comment above still relevant as a possible fix

[bkamins]

Ah - you are right. I thought nonmissingtype_checked was changed. Sorry for the confusion.

[o314]

Label "types and dispatch" should be applied there