Right now, Julia will happily do:

julia> convert(Ptr{Int}, [1.0])
Ptr{Int64} @0x000000010367e470

This is an invitation for bugs; I just ran into this today with a student who was doing ccall(:foo, Void, (Ptr{Cint},), x) and didn't understand why it was failing for x = [1,2,3] or x = [4.0, 5.0].

My preference would be to throw an error in this case. We can still allow convert(::Type{Ptr{T1}}, x::Ptr{T2}) to work, so if you want to do an explicit cast from one pointer type to another you can still do so.