# @generated function type inference failure

**URL:** <https://discourse.julialang.org/t/generated-function-type-inference-failure/2796>\
**Category:** General Usage\
**Created:** [March 21, 2017, 4:43pm UTC](https://discourse.julialang.org/t/generated-function-type-inference-failure/2796 "2017-03-21T16:43:58Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![marius311](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marius311/32/3953_2.png) [@marius311](https://discourse.julialang.org/u/marius311)\
**Post date:** [March 21, 2017, 4:43pm UTC](https://discourse.julialang.org/t/generated-function-type-inference-failure/2796/1 "2017-03-21T16:43:58Z")

</div>

I noticed that type inference on the following fairly simple code fails, but works if you just remove the @generated. I’m wondering if this is a known issue? It appears present both on 0.5 and master. If its a bug, does anyone have any workarounds? Thanks.

```julia
julia> struct MyType{T} end

julia> foo() = baz([MyType{1}(),MyType{2}()][1])
foo (generic function with 1 method)

julia> @generated function baz(a::MyType) 1 end
baz (generic function with 1 method)

julia> @code_warntype foo()
Variables:
  #self#::#foo
  T::Any

Body:
  begin 
      SSAValue(0) = (Core.tuple)($(QuoteNode(MyType{1}())), $(QuoteNode(MyType{2}())))::Tuple{MyType{1},MyType{2}}
      $(Expr(:inbounds, false))
      # meta: location array.jl vect 70
      SSAValue(1) = (Base.nfields)(SSAValue(0))::Int64
      # meta: pop location
      $(Expr(:inbounds, :pop))
      SSAValue(3) = $(Expr(:invoke, MethodInstance for copy!(::Array{MyType,1}, ::Tuple{MyType{1},MyType{2}}), :(Base.copy!), :($(Expr(:foreigncall, :(:jl_alloc_array_1d), Array{MyType,1}, svec(Any, Int64), :($(QuoteNode(Array{MyType,1}))), 0, SSAValue(1), 0))), SSAValue(0)))
      return (Main.baz)((Base.arrayref)(SSAValue(3), 1)::MyType)::Any
  end::Any

```

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<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [March 21, 2017, 5:13pm UTC](https://discourse.julialang.org/t/generated-function-type-inference-failure/2796/2 "2017-03-21T17:13:23Z")

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It does not come from generated functions. You constructed an array with non-leaf type. The type inference does not do constant propagation across array construction/loading.

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<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [March 21, 2017, 5:15pm UTC](https://discourse.julialang.org/t/generated-function-type-inference-failure/2796/3 "2017-03-21T17:15:29Z")

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And it works without generated function because the compiler can infer the return type/value with non-leaftype input. For generated function, it is impossible (theoritically) to do that. This is a reason to only use generated function when necessary and expect worse performance with non-leaf type info.

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<div class="post-metadata">

**Author:** ![marius311](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/marius311/32/3953_2.png) [@marius311](https://discourse.julialang.org/u/marius311)\
**Post date:** [March 21, 2017, 5:20pm UTC](https://discourse.julialang.org/t/generated-function-type-inference-failure/2796/4 "2017-03-21T17:20:46Z")

</div>

Interesting, thanks for the explanation, I think that made sense. I didn’t know about this limitation of @generated functions before.