Inhibit same type parameters

General Usage
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2

There’s no way to make it so that simply writing P{A, A} errors, if that’s what you want. If you call dump(P), you get

julia> dump(P)
UnionAll
  var: TypeVar
    name: Symbol F
    lb: Core.TypeofBottom Union{}
    ub: Union{A, B, C, D}
  body: UnionAll
    var: TypeVar
      name: Symbol S
      lb: Core.TypeofBottom Union{}
      ub: Union{A, B, C, D}
    body: P{F<:Union{A, B, C, D},S<:Union{A, B, C, D}} <: Any
      a::Array{T,1} where T
      b::Array{T,1} where T

This shows how UnionAlls like P are represented in Julia. You can have upper bounds (ub) and lower bounds (lb) on type variables, but UnionAll itself does not support more complicated constraints, probably to limit the computational complexity of type system queries (but I’m far from an expert on this subject).

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