filter() function edge case, possible bug.

[ericproffitt]

Consider the code:

filter(n -> true, collect(1:0))

this returns a 0-element array. However if I don't collect the iterator into an array:

filter(n -> true, 1:0)

I get the error

Error: TypeError: typeassert: expected AbstractArray{Bool, N}, got Array{Int64, 1} in filter at array.jl:923

It seems like this should also return a 0-element array, since if the iterator is non empty, e.g.

filter(n -> true, 1:2)

I get the two element array [1,2].

[ararslan]

I can reproduce this on 0.4.5 but on 0.5 the error is slightly different: it says Array{Union{},1} instead of Array{Int64,1}.

[berndbohmeier]

In array.jl the function filter is implemented using map
filter(f, As::AbstractArray) = As[map(f, As)::AbstractArray{Bool}]

but map does not change the elementtype of the AbstractArray if the AbstractArray is empty.

map(n-> true, 1:2) results in Array{Bool,1}
but
map(n-> true, 1:0) results in Array{Int64,1} , which causes the problem in filter

Is this the expected behaviour of map? Edit: seems to be related to #210, map infers the type from the first element, so it can not get the right type.