# Order dependence of calls to isa through Cassette

**URL:** <https://discourse.julialang.org/t/order-dependence-of-calls-to-isa-through-cassette/44259>\
**Category:** General Usage\
**Created:** [August 4, 2020, 1:56pm UTC](https://discourse.julialang.org/t/order-dependence-of-calls-to-isa-through-cassette/44259 "2020-08-04T13:56:13Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![ckfinite](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ckfinite/32/8341_2.png) [@ckfinite](https://discourse.julialang.org/u/ckfinite)\
**Post date:** [August 4, 2020, 1:56pm UTC](https://discourse.julialang.org/t/order-dependence-of-calls-to-isa-through-cassette/44259/1 "2020-08-04T13:56:13Z")

</div>

I’m having a bit of a weird problem with Cassette when trying to overdub isa , as the result appears to change based on the sequence of calls despite no explicit state being shared between the calls. The example

```julia
using Cassette
Cassette.@context Ctx

typ = Tuple{P} where P <: Int
a = Cassette.overdub(Ctx(), isa, typ, DataType)
b = Cassette.overdub(Ctx(), isa, typ.body, DataType)
println("false: $a true: $b")

```

produces `false: false true: false` despite the expected result being `false: false true: true`. If a is replaced by a constant, then b is correct (is true); if we invoke isa with Cassette.fallback it works, and if we replace the definition of Cassette.call with one that calls jl\_isa directly then it works. For example, adding

```julia
function Cassette.call(ctx::Cassette.Context{Cassette.nametype(Ctx),M,Nothing,P,B,H} where H<:Union{Cassette.DisableHooks, Nothing} where B<:Union{Nothing, IdDict{Module,Dict{Symbol,Cassette.BindingMeta}}} where P<:Cassette.AbstractPass where M, ::typeof(isa), a, b::Type) 
	return ccall(:jl_isa, Cint, (Any, Any), a, b) == 1
end

```

in between the definition of Ctx and the calls to overdub fixes the problem.

I suspect that the proximal cause of the issue is something to do with the unbound type variable in `typ.body` causing the second invocation to call the older (yet still type-specialized) version of `isa`, as when you attempt to identify the specialization that is called specifically reflection yells at you because it is a non-leaf-type. However, I haven’t been able to reproduce this in other generated methods, so am at something of a loose end as to where to look next.

---

<div class="post-metadata">

**Author:** ![ckfinite](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ckfinite/32/8341_2.png) [@ckfinite](https://discourse.julialang.org/u/ckfinite)\
**Post date:** [August 5, 2020, 10:07pm UTC](https://discourse.julialang.org/t/order-dependence-of-calls-to-isa-through-cassette/44259/2 "2020-08-05T22:07:59Z")

</div>

I’ve narrowed this down to a bug in generated functions, see here [https://github.com/JuliaLang/julia/issues/36931](https://github.com/JuliaLang/julia/issues/36931)