v0.5 "cannot add methods to an abstract type" when overriding call

I'm updating ApproxFun and this caught me by surprise:

``` julia
abstract Foo
julia> (f::Foo)(x)=x
ERROR: cannot add methods to an abstract type
 in eval(::Module, ::Any) at ./boot.jl:267
```

Is this intentional?  Is there a way to get around this other than define for each subtype:

``` julia
julia> immutable CFoo <: Foo end
julia> (f::CFoo)(x)=x
```
