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openedon Mar 14, 2018
If I understand correctly, right now the rule for Tuple{T,T} where T>:t
is: T
is considered diagonal unless it is obvious that t
is abstract. Therefore, in the examples below the types on the right are not diagonal:
julia> (Tuple{A,A} where A>:Integer) <: Tuple{T,T} where T>:Number
true
julia> (Tuple{A,A,Number} where A>:Number) <: Tuple{T,T,S} where T>:S where S>:Integer
true
In the following modification of the previous example, however, T
is considered diagonal despite the fact that its lower bound S
can be abstract.
julia> (Tuple{A,A,Number} where A>:Number) <: Tuple{T,T,S} where T>:S where S
false
I totally see that it is a hard task to determine concreteness of types involving variables. What I would suggest is to change the strategy for Tuple{T,T} where T>:t
and consider T
non-diagonal unless it is obvious that t
is concrete. I think that if programmers write lower bounds, it is likely that they really want to switch off the diagonal rule.
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