Trying to build recursive types to annotate a nested data structure, I hit the following.

This code is correct according to mypy:

IntType = int | list["IntType"] | tuple["IntType", ...]
StrType = str | list["StrType"] | tuple["StrType", ...]


def int2str(x: IntType) -> StrType:
    if isinstance(x, list):
        return list(int2str(v) for v in x)
    if isinstance(x, tuple):
        return tuple(int2str(v) for v in x)
    return str(x)

But not this one, which should be equivalent:

IntType = int | list["IntType"] | tuple["IntType", ...]
StrType = str | list["StrType"] | tuple["StrType", ...]

def bad_int2str(x: IntType) -> StrType:
    if isinstance(x, (list, tuple)):
        return type(x)(bad_int2str(v) for v in x)  # error here
    return str(x)

The error message is

line 6: error: Incompatible return value type (
    got "list[int | list[IntType] | tuple[IntType, ...]] | tuple[int | list[IntType] | tuple[IntType, ...], ...]", 
    expected "str | list[StrType] | tuple[StrType, ...]"
)  [return-value]
line 6: error: Generator has incompatible item type 
    "str | list[StrType] | tuple[StrType, ...]"; 
    expected "int | list[IntType] | tuple[IntType, ...]"  [misc]

I would assume mypy could infer that type(x) is either list or tuple.
Is this a limitation of mypy or is there something fishy with this code?
If so, where does the limitation come from?