Libtask v0.9.9 breaks Union types

[penelopeysm]

This is causing Turing PG/SMC to fail on all models.

using Libtask

g(i::Int) = begin; produce(1); i; end
g(s::String) = begin; produce(2); s; end
Libtask.@might_produce g

h(i::Int)::Int = i
h(s::String)::Int = length(s)

function f(x)
    i_or_s = if x > 0
        3
    else
        "libtask"
    end
    # These lines are just to make sure that `f` itself is type stable.
    # You can trigger the same error by just returning `g(i_or_s)`
    gres = g(i_or_s)
    return h(gres)
end

t = TapedTask(nothing, f, 1)
consume(t) # returns 1
consume(t) # should return `nothing`, but errors on 0.9.9 with:

#=
ERROR: TypeError: in typeassert, expected Base.RefValue{Union{Int64, String}}, got a value of type Base.RefValue{Int64}
Stacktrace:
 [1] deref_phi
   @ ~/ppl/Libtask.jl/src/copyable_task.jl:1229 [inlined]
 [2] f
   @ ./REPL[26]:2 [inlined]
 [3] (::Tuple{…})(_2::typeof(f), _3::Int64)
   @ Base.Experimental ./<missing>:0
 [4] consume(t::TapedTask{Nothing, Tuple{typeof(f), Int64}, MistyClosures.MistyClosure{Core.OpaqueClosure{…}}})
   @ Libtask ~/ppl/Libtask.jl/src/copyable_task.jl:378
 [5] top-level scope
   @ REPL[29]:1
=#

It works fine on 0.9.8 so this is caused by #205. You can fix it by moving the call to gres inside the if/else, so this works:

function f(x)
    gres = if x > 0
        g(3)
    else
        g("libtask")
    end
    return h(gres)
end

The immediate Turing breakage is here. You can work around it by moving the tilde_assume!! call inside the if/else just like with g, but doing that just shifts the error to something else in DynamicPPL, I think from_maybe_linked_vec_transform.

[Technici4n]

Nice reproducer! Indeed we have to be a little more careful with union types it seems. (We need a Union of RefValues rather than a RefValue of Unions here)

[mhauru]

This is my bad, I got too used to the Turing integration tests failing and didn't even check if the failure was the expected one. Now waiting for, and expecting, them to pass for #209, and will the release the fix.