I ran into the following when tracking down a type instability issue.
Consider the following type hierarchy:
```julia
type T1{I<:Number} end
type T2{T<:T1}
x::T
end
type T3{T<:T2}
x::T
end
type T4{T<:T3}
x::T
end
type T5{T<:T4}
x::T
end
```
Now, two functions, one which returns a `T4`, and one which returns a `T5`.
```julia
function foo()
t2 = T2( T1{Float64}() )
t3 = T3( t2 )
T4( t3 )
end
function bar()
t2 = T2( T1{Float64}() )
t3 = T3( t2 )
t4 = T4( t3 )
T5( t4 )
end
```
Julia is able to infer the return type of `foo`, but not of `bar`.
```
julia> @code_warntype foo()
Variables:
t2::T2{T1{Float64}}
t3::T3{T2{T1{Float64}}}
Body:
begin #
t2 = $(Expr(:new, T2{T1{Float64}}, :($(Expr(:new, T1{Float64}))))) #
line 22:
t3 = $(Expr(:new, T3{T2{T1{Float64}}}, :(t2::T2{T1{Float64}}))) #
line 23:
return $(Expr(:new, T4{T3{T2{T1{Float64}}}},
:(t3::T3{T2{T1{Float64}}})))
end::T4{T3{T2{T1{Float64}}}}
Variables:
t2::T2{T1{Float64}}
t3::T3{T2{T1{Float64}}}
t4::T4{T3{T2{T1{Float64}}}}
julia> @code_warntype bar()
Body:
begin #
t2 = $(Expr(:new, T2{T1{Float64}}, :($(Expr(:new, T1{Float64}))))) #
line 28:
t3 = $(Expr(:new, T3{T2{T1{Float64}}}, :(t2::T2{T1{Float64}}))) #
line 29:
t4 = $(Expr(:new, T4{T3{T2{T1{Float64}}}},
:(t3::T3{T2{T1{Float64}}}))) # line 30:
GenSym(0) = t4::T4{T3{T2{T1{Float64}}}}
return
((top(apply_type))(T5,T4{T3{T2{T1{Float64}}}})::TYPE{_<:T5{T<:T4{T<:T3{T<:T2{T<:T1{I<:NUMBER}}}}}})(GenSym(0))::T5{T<:T4{T<:T3{T<:T2{T<:T1{I<:NUMBER}}}}}
end::T5{T<:T4{T<:T3{T<:T2{T<:T1{I<:NUMBER}}}}}
```
Am I running up against a genuine limit in Julia's type inference?
