void main() {
    uint[][] foo = [[1, 2], [3, 4], [5, 6]];
    immutable bar = foo.idup;
}

Compiler error:

test.d(3): Error: undefined identifier module test.idup
test.d(3): Error: function expected before (), not module test.idup of type void

Of course, the compiler should not allow .idup to be applied to foo because, unless a deep copy were made, this would cause unsafe aliasing between mutable and immutable.  However, a better error message might be a good thing.