Dispatch error with bound TypeVar argument on a second call to a "recursive" function.

[yuyichao]

Repro:

function f{T<:Tuple}(sig::Type{T})
    println(1)
    f(sig.parameters[1])
end
function f(arg::DataType)
    println(2)
    if arg <: Tuple
        error("Dispatch error!")
    end
end

t = Tuple{Type{Dict{TypeVar(:K, true)}}}

println("First time")
f(t)
println("Second time")
f(t)

Mailing list thread
@mauro3

[yuyichao]

Output:

yuyichao% JULIA_LOAD_PATH=${PWD}/.. julia -f runtests.jl
First time
1
2
Second time
2
ERROR: LoadError: Dispatch error!
 in f at /home/yuyichao/projects/mirrors/Traits.jl/runtests.jl:10
 in include_from_node1 at ./loading.jl:134
while loading /home/yuyichao/projects/mirrors/Traits.jl/runtests.jl, in expression starting on line 19

Changes that can make the problem disappear:

1. Do not call f(::DataType) in f{T<:Tuple}(::Type{T})
2. Make the TypeVar not bound
3. Call f only once
4. (Added by @mauro3 , see next comment) Remove ::DataType in the definition of the second method

Maybe some caching issue??

[mauro3]

Leaving away ::DataType in function f(arg::DataType) makes the problem disappear too.

[yuyichao]

Yeah you are right. Forgot to check that.

[mauro3]

Although, in Traits.jl that trick doesn't seem to work (a shame, I thought I had a work-around).

[yuyichao]

You have another definition below that rigth?

[mauro3]

Yes, but adding that other definition doesn't change the behavior of your example. It doesn't matter though, there are other workarounds.

[mauro3]

This test case still errors in the last line:

function f{T<:Tuple}(sig::Type{T})
    println(1)
    f(sig.parameters[1])
end
function f(arg::DataType)
    println(2)
    if arg <: Tuple
        error("Dispatch error!")
    end
end

t = Tuple{Type{Array{Int,1}}}
f(t)
t = Tuple{Type{Dict{TypeVar(:K, true)}}}
f(t)

with

2
ERROR: Dispatch error!
 in f at none:4

Please re-open.