Check Type has parameters field with an element

[Zentrik]

Fixes #491, #492, #494

[Zentrik]

Unfortunately it seems that extract_inner_type(::Type{Type{T}}) specializes on each new type it sees, I don't know how problematic that is. Also not sure why we were taking the first element of T.parameters before but this implementation seems to work fine.

[codecov-commenter]

Codecov Report

Merging #493 (30ec350) into master (a017a4d) will not change coverage.
The diff coverage is n/a.

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##           master    #493   +/-   ##
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  Coverage    0.00%   0.00%           
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  Files           9       9           
  Lines        1511    1511           
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  Misses       1511    1511           

[timholy]

I doubt it will be measurable (and so we can merge this), but presumably extract_inner_type will be specialized for every type. The old code tried to avoid that but it may not be readily fixable. So I'm fine with merging this, thanks for tackling it!

EDIT: Oh, just saw you commented on the same issue.

[timholy]

Hmm, I guess this should have been T <: Type{S} where S or something?

[Zentrik]

Union{} <: Type{S} where S is true so I don't think that would have fixed it either.

[vtjnash]

It should be T isa DataType or Base.isType(T)

[timholy]

Also not sure why we were taking the first element of T.parameters before but this implementation seems to work fine.

julia> TT = Type{Float64}
Type{Float64}

julia> TT.parameters[1]
Float64

Basically we were extracting the T in the same way as your extract_inner_type, without forcing dispatch specialization. I think the problem with the old implementation is that T <: Type was too weak of a condition. Perhaps T <: Type{S} where S might fix it?

[Zentrik]

I thought extract_inner_type(@nospecialize(x::Type{<:Type})) = x.parameters[1] might work, but unfortunately it fails with Core.TypeofBottom. Given I don't know when a type actually has a non empty parameters svec might be easiest to just use this implementation.

[simeonschaub]

I believe it should be enough to just change the check to T isa DataType && T <: Type

[Zentrik]

Core.TypeofBottom satisfies that but has an empty parameters svec, maybe checking if it's non empty as well will work.

[Zentrik]

Given we don't think the current implementation will be particularly slow I'll merge it for now, and we can revisit this if it is slow.