Confusing ambiguous MethodError

General Usage
1

While using SQLite.jl, I’ve run into this error (but I don’t think it is really a SQLite question.)

MethodError: promote_type(::Type{Union{}}, ::Type{String}) is ambiguous. Candidates:
  promote_type(::Type{Union{}}, ::Type{T}) where T in Base at promotion.jl:280
  promote_type(::Type{T}, ::Type{String}) where T<:WeakRefStrings.InlineString in WeakRefStrings at C:\Users\nritchie\.julia\packages\WeakRefStrings\a3jYm\src\inlinestrings.jl:44
Possible fix, define
  promote_type(::Type{Union{}}, ::Type{String})

If I understand correctly, Union{} is an empty union of types.
So the first promote candidate promote_type(::Type{Union{}}, ::Type{T}) where T is saying that when selecting between type no type or type T, always promote to type T.
The second candidate says when selecting between an InlineString and a String, promote to String.
Now, given this, I don’t understand why the second candidate is being proposed for promote_type(::Type{Union{}}, ::Type{String}). As best I can tell, ::Type{Union{}} does not match ::Type{<:InlineString}.

What am I missing?

2

in the second method, the second argument is fixed to Type{String}, the T <: Inline applies to the first argument

3

Ok, I agree that the second argument is fixed to ::Type{String} but how does ::Type{T} where T<: InlineString match the first argument ::Type{Union{}}?

If I do define a new
promote_type(::Type{Union{}}, ::Type{String}) = String
everything works but I don’t think I should need to.

4

The defining property of Union{} is that Union{} <: T is true for all types T.

That said, you should never overload promote_type directly, instead overload promote_rule for your custom type.

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