uncastable

That cannot be cast (filled with, or allocated, a theatrical role).

Colleen Dewhurst, though in many ways wrong for the virtually uncastable Josie, is as lucid and luminous […]

[…] that he would, like many actors, become so stereotyped in the public mind as to be uncastable in other roles […]

That cannot be cast (converted to other data types).

They're strongly typed, uncastable pointers […]

That cannot be set in a cast.

...internal fixation in children with open, unstable, or otherwise uncastable forearm fractures […]

That cannot be used for, or produced through, casting (manufacturing with a mould).

Casting must be done within a day or two of moulding or the moulds will begin to wind-dry and become uncastable […]

This allows the casting of otherwise uncastable materials, with acceptable rejection rates, and can increase ductility by as much as a factor of six.