_SubClassValueT is only useful when it is has >1 usage in a signature.
This was not true for the signatures produced by tblgen.
For example
def call(result, callee, operands_, *, loc=None, ip=None) ->
_SubClassValueT:
...
here a type checker does not have enough information to infer a type
argument for _SubClassValueT, and thus effectively treats it as Any.