Unexpected method dispatch with parametric type constructor

[danielmatz]

In the following example:

julia> struct Foo{T <: Integer}
    x::T
    y::T
end

julia> struct Bar{T}
    x::Foo{T}
    y::Foo{T}
end

julia> methods(Bar)
# 1 method for type constructor:
[1] Bar(x::Foo{T}, y::Foo{T}) where T in Main

julia> Bar(x::Foo, y::Foo) = "generic"

julia> methods(Bar)
# 2 methods for type constructor:
[1] Bar(x::Foo, y::Foo) in Main
[2] Bar(x::Foo{T}, y::Foo{T}) where T in Main

julia> Bar(Foo(1, 2), Foo(1, 2))
"generic"

I had expected the call to Bar to dispatch to the Julia-provided method Bar(::Foo{T}, ::Foo{T}) where T, since the argument types are the same.

In other situations, dispatch does behave according to this expectation. For example:

# Using `Foo` from above

julia> Foo(x::Integer, y::Integer) = "generic"

julia> Foo(1, 2)
Foo{Int64}(1, 2)

julia> Foo(Int8(1), 2)
"generic"

We discussed this on Discourse.