Potential lowering bug on 1.12

[Drvi]

As spotted by @NHDaly on RelationalAI slack; I'm making this issue so it doesn't get lost.

function assign_node_ids(node, starting_id::Int)

    ii = starting_id

    function _traverse(φ::BackIRType)
        new_φ = _copy_and_assign_id(φ, ii)
        if new_φ !== φ
            ii += 1
        end
        return new_φ
    end

    _traverse(n) = n

    return (bottomup(_traverse)(node), ii)
end

Which gave

        UndefVarError: `ii` not defined in local scope
        Suggestion: check for an assignment to a local variable that shadows a global of the same name.

pointing to the first line in the closure:

        new_φ = _copy_and_assign_id(φ, ii)

CC: @gbaraldi

[NHDaly]

Thanks Tomas. @gbaraldi you have access to our codebase and this should be easy to reproduce if you want to! We can even walk you through it again on zoom if you want

[KristofferC]

I tried the following (just naively define the undefined types and functions):

g(a,b) = b
bottomup(x) = x
struct BackIRType end

function assign_node_ids(node, starting_id::Int)
    ii = starting_id
    function _traverse(φ::BackIRType)
        new_φ = g(φ, ii)
        if new_φ !== φ
            ii += 1
        end
        return new_φ
    end

    _traverse(n) = n

    return bottomup(_traverse)(node), ii
end

assign_node_ids(BackIRType(), 1)

and it doesn't reproduce on 1.12-nightly.

julia> assign_node_ids(BackIRType(), 1)
(1, 2)

So either this has been fixed or there is something special about the actual types and functions used in the original example (which is a bit surprising since they shouldn't affect things like what variables are defined).

[nickrobinson251]

#57141

[nickrobinson251]

fixed by #57201