promoting Nullable with anything?

[KristofferC]

Moved from JuliaLang/julia#19748

---

e79e84654a40e8189e5dfbd55df232ae6c82a213 added the following definition:

promote_rule{S,T}(::Type{Nullable{S}}, ::Type{T}) = Nullable{promote_type(S, T)}

This might have unintended consequences. For example, we have:

julia> promote(1, "")
(1,"")

julia> promote(Nullable(1), "")
(Nullable{Any}(1),Nullable{Any}(""))

Given a Nullable and something completely unrelated, you get two Nullable{Any}s. I don't think two types should have a non-identity promotion just because one is Nullable. This was not obvious at the time.

I propose this definition instead:

function promote_rule{S,T}(::Type{Nullable{S}}, ::Type{T})
    R = promote_type(S, T)
    if R <: S || R <: T
    else
        if S <: R || T <: R
            return R
        end
    end
    return Nullable{R}
end

This says to leave the values alone if the promoted type (R) is a strict supertype of T or S, and otherwise promote as before. For example

julia> promote(Nullable(1),2.4)
(Nullable{Float64}(1.0),Nullable{Float64}(2.4))

julia> promote(Nullable(1),"")
(Nullable{Int64}(1),"")

This definition can still be type-inferred (since we can now handle constant conditions well).