Julia: [Julia v1.30-rc4.1] convert(::Type{Missing}, ::Int64) is ambiguous

[rapus95]

julia> convert(Missing, 5)
ERROR: MethodError: convert(::Type{Union{}}, ::Int64) is ambiguous. Candidates:
  convert(::Type{Union{}}, x) in Base at essentials.jl:166
  convert(::Type{T}, x::Number) where T<:Number in Base at number.jl:7
  convert(::Type{T}, arg) where T<:VecElement in Base at baseext.jl:8
  convert(::Type{T}, x::Number) where T<:AbstractChar in Base at char.jl:179
Possible fix, define
  convert(::Type{Union{}}, ::Number)
Stacktrace:
 [1] convert(::Type{Missing}, ::Int64) at .\missing.jl:69
 [2] top-level scope at REPL[240]:1

[KristofferC]

I saw this too a while ago and I remember thinking that the

  convert(::Type{T}, arg) where T<:VecElement in Base at baseext.jl:8

seems kinda fishy. It feels like it could just be removed.

[rapus95]

Why there is no convert(::Type{Missing}, arg) with some fancy error like error("haha, forget it (pun intended)") if !ismissing(arg)? /sarcasm:off

[rapus95]

Well, my sarcasm and pun (convert to missing <=> forget) feels a bit displaced now, sorry for that. What I meant, was, why there is no specialized version for convert(::Type{Missing}, x)? Aka, why is convert to Missing the same as a convert to Union{}?

[KristofferC]

Because of

https://github.com/JuliaLang/julia/blob/570b508690dbdec9f28cca13f5c47682be95cd9d/base/missing.jl#L69

and

https://github.com/JuliaLang/julia/blob/570b508690dbdec9f28cca13f5c47682be95cd9d/base/missing.jl#L21-L39

[Keno]

I see two solutions:

1. Define:

convert(::Type{Missing}, x) = missing

1. Define

convert(::Type{Missing}, x) = throw(MethodError(convert, (Missing, x)))

Which do we want?

[KristofferC]

Surely not the first? 😰