type synonym required to avoid "polymorphic type Data[T] illegal here" error

[GoogleCodeExporter]

In example code shown below variables "b" and "c" assigned to same thing, but 
assignment to "b" fail with compile with "polymorphic type Data[T] illegal 
here" error. Needs to use "type" synonym just to work around this error is 
annoying. :(

implement Deeppoly;

include "sys.m";
include "draw.m";

Deeppoly: module
{
    init: fn(nil: ref Draw->Context, nil: list of string);
};

Data: adt[T] {
    new: fn(): ref Data[T];
};

Data[T].new(): ref Data[T]
{
    return ref Data[T]();
}

init(nil: ref Draw->Context, nil: list of string)
{
    a := Data[string].new();

    # error: polymorphic type Data[T] illegal here
    b := Data[ref Data[string]].new();

    DataString: type Data[string];
    c := Data[ref DataString].new();
}

Original issue reported on code.google.com by powerman...@gmail.com on 3 Apr 2013 at 1:56