"No method" error with parametric type Union(Nothing, T)

[nalimilan]

This is similar to #3738 but it also affects positional arguments.

julia> test{T<:Number}(x::Union(Nothing, T)) = true
test (generic function with 1 method)

julia> test(1)
true

julia> test(nothing)
ERROR: no method test(Nothing)

[JeffBezanson]

This behavior is correct. A parametric method cannot be called unless values for the parameters can be determined. For example

julia> f{T}(x) = 0
Warning: static parameter T does not occur in signature for f at none:1.
The method will not be callable.

However we can't give the warning in this case since it is possible to call the method.

[nalimilan]

But it means unions with Nothing and any parametric type cannot be used, and in particular that Union(Nothing, AbstractArray{T}) with T<:Real does not work. This is annoying since there's no way of passing an equivalent of NULL for an argument. See my use case (but for keyword arguments) at https://groups.google.com/d/msg/julia-users/FAKPYxxiQgQ/VFW9l6-tqF8J

Wouldn't it be possible to call the method even if T is not specified, since nothing matches a type in the union?