For example, dict[int, int] cannot be used with singledispatch because types.GenericAlias doesn't support weak references.

I'm an author of a third party library (https://github.com/Tinche/cattrs) for which this functionality would be useful.

Here's a similar issue in typing (note that this issue is for *types*.GenericAlias) that was fixed: https://github.com/python/typing/issues/345