type parameter UndefVarError when dispatch matches UnionAll

[goretkin]

This behavior surprised me:

sometype = Array{Tuple{R}} where {R}

something(::Type{T}) where {T <: Array{S}} where {S<:Tuple} = S

something(sometype) # gives ERROR: UndefVarError: S not defined

I am not sure if I expected S to be bound to Tuple{R} where {R} or for a MethodError. But either way, I was not expecting S to be ever undefined if it shows up in a where.

versioninfo:

julia> versioninfo()
Julia Version 1.1.0-DEV.318
Commit 993e7d7733 (2018-09-25 06:36 UTC)

[JeffBezanson]

This is intentional; see #23117. The type is a subtype of the signature, so the method should match. But there is no unique correct value for S.

[goretkin]

Thanks for the explanation. I ran into this when using the Size trait of StaticArrays.jl (e.g. Size(SVector)). Looks like I wouldn't have run into it on the master branch of StaticArrays: JuliaArrays/StaticArrays.jl@bfa5536