How to dispatch on a type alias?

[Benny]

@mbauman

I think that’s just a rule of how dispatch with type parameters works — it requires matching against a fully-resolved type.

I’ve been thinking of it in my head as method where clauses requiring the parameters be specified because they may be matched across multiple arguments or be accessible values in the method body. Reminds me of this past thread, I’ll just share the last small example sans the thread’s context:

@Benny

Incidentally, this is where it is important to specify parameters belonging to the argument versus the method. A version of the correct code for the number of dimensions would look like

my_ndims(::Type{<:AbstractArray{T,N} where T}) where {N} = N

my_ndims(Union{Array{Int,1}, Array{Bool,1}}) # 1

Base.ndims now behaves like this, but it used to error on unspecific T like this Union or Array{T,1} where T because its method had both parameters {T,N} (issue #40682 ).

As for this particular case:

# works for _(C), fails for _(C{Int, Float64})
function myfunction(::Type{ A{T1,B{T3}} where {T1,T3} })
  return 1
end

# names disambiguated to avoid multimethods
# works for _(C{Int, Float64}), fails for _(C)
function myfunction1(::Type{ A{T1,B{T3}} }) where {T1,T3}
  return 1
end

function myfunction2(::Type{ A{T1,B{T3}} } where {T1, T3})
  return 1
end

As said before, method parameters must be specified, so myfunction1 failing makes sense. myfunction2 fails because the parametric Type requires that the T1 and T3 be specified, in the same way that D = Vector{Ref{T}} where T only includes Vector{Ref{Int}} or Vector{Ref{Float64}}, but not Vector{Ref}.

In fact myfunction1 and myfunction2 would override each other with the same function name because the argument requires specified T1 and T3, despite not being equivalent (T1 and T3 are not available in the method body nor can they matched across arguments).