call signature with free type variable can corrupt the dispatch cache

[KristofferC]

MWE:

using SciMLBase

function check_isdefined(::Type{<:SciMLBase.IntervalNonlinearFunction{iip}}) where {iip}
    @show @isdefined iip
    if @isdefined iip
        @show iip
    end
end

dt2 = IntervalNonlinearFunction{TypeVar(:iip)}

check_isdefined(dt2)

On bad Julia versions this gives

#= /home/kristofferc/julia/bisect.jl:9 =# @isdefined(iip) = true
ERROR: LoadError: UndefVarError: `iip` not defined

On good ones

#= /home/kristofferc/julia/bisect.jl:9 =# @isdefined(iip) = false

Bisected to #46791 (which was backported). cc @aviatesk

Xref: #35661

[vtjnash]

Note that this construction will partially corrupt the dispatch cache currently (even after #49097), potentially leading to undefined behavior, so users should avoid defining methods that may get called like that for now. We have a few open issues about eventually fixing that, but it has not been a priority yet.

[KristofferC]

I am not sure this exact construction is what is used in practice. It was more something like:

Base.show(io::IO, t::Type{A{X, Y}} where {X, Y}
    if @isdefined X
        print(io, "Type{X, ...}")
    else
        @invoke show(io::IO, t::Type)
    end
end

where X was not defined (but the @isdefined branch was taken).

[vtjnash]

Yes, that is what is not permitted currently. Since show normally dispatches on kinds, overloading show for a type will introduce undefined behavior into your application.