method_exists not working with TypeVar-parameterized types

[mauro3]

Using TypeVars for non-parameterized types works with method_exists:

julia> I = TypeVar(:X, Integer)
X<:Integer

julia> Ii = TypeVar(:X, Int)
X<:Int64

julia> g{I<:Integer}(x::I) = x
g (generic function with 1 method)

julia> method_exists(g, (Integer,))
false

julia> method_exists(g, (Int,))
true

julia> method_exists(g, (I,))
false

julia> method_exists(g, (Ii,))
true

(Edit: Actually, shouldn't the second last return true as the method does exist for all <:Integer?)

But not for parameterized types:

julia> f{I<:Integer}(x::Array{I,1}) = x[5]
f (generic function with 1 method)

julia> method_exists(f, (Vector{Integer},))
true

julia> method_exists(f, (Vector{Int},))
true

julia> method_exists(f, (Vector{I},))
false

julia> method_exists(f, (Vector{Ii},))
false

I would have thought the latter two should return true.

[JeffBezanson]

Covered by #8974. Also please do not call TypeVar manually. It should probably not be exported, come to think of it.

[mauro3]

I'm using this over in Traits.jl, see mauro3/Traits.jl#2. I think, to store the signature of a parameterized function in a trait definition a TypeVar is needed as simple types cannot store that kind of subtype relationship. But that may change with #6984 & #8974.

[JeffBezanson]

It will need to change. I cannot figure out how to make the system work with unrestricted use of TypeVars. For example:

julia> T=TypeVar(:T)
T

julia> isa(T,Type{T})
true

julia> isa(Int,Type{T})
true

julia> Int==T
false

This breaks the rule that isa(x,Type{y}) <=> (x == y). In the future we will have isa(Int, @UnionAll T Type{T}), but not Type{T} by itself.

[mauro3]

Thanks for this clarification. I'm looking forward to #8974 sorting this all out!