Method sorting with restricted/unrestricted type parameter (extracted from #6383)

[iamed2]

A separate issue for @mbauman's #6383 (comment) which is distinct from the ambiguity problem.

julia> struct Foo{A<:Real} end

julia> f(x::Foo{A}) where A = 1
f (generic function with 1 method)

julia> f(x::Foo, y...) = 2
f (generic function with 2 methods)

julia> f(Foo{Int}())
2

The second method's 1-arg form appears (unless you're an Expert) as if it is less specific than the first. However, Foo is treated as Foo{T} where T<:Real, while the compiler puts a type bound of Any on A.

I think it would make sense if Foo{A} where A was treated as Foo{A} where A<:Real, at least in method definitions.

See also JuliaArrays/AxisArrays.jl#124 (comment)

[JeffBezanson]

The original issue isn't really specific to ambiguities; the underlying issue is exactly the same whether you get an ambiguity or an unexpected sort order.