3.x: Mypy Unreachable

[deckar01]

The unreachable check was disabled in #2878 after https://github.com/marshmallow-code/marshmallow/actions/runs/20423614307/job/58679701675 blocked the pypi 3.36.2 security release.

src/marshmallow/fields.py:627: error: Statement is unreachable [unreachable]

[deckar01]

When I remove the offending type cast it reveals the original error the cast silenced:

src/marshmallow/fields.py:534: error: Incompatible types in assignment (expression has type "Schema | str | dict[str, Field[Any]] | type", variable has type "Schema | SchemaMeta | dict[str, Field[Any]]") [assignment]

The argument hint is missing str in the callable return type. The code correctly finds the class from a callable that returns a string, so that is an easy fix.

The other difference is that mypy turned SchemaMeta into type in the argument type at the top level, but not in the return type of the callable.type seems to be caused by the not isinstance(..., type) check shadowing SchemaMeta as an alias for type. mypy can't implicitly union the branched types here, so it assumes the type in the first branch and complains about the second assignment not being compatible with it. Possibly a mypy bug. The proper fix is to nested = self.nested to establish the broadest type, and the callable check safely narrows it.

not isinstance(..., type) can then be switched to not isinstance(..., SchemaMeta) to clarify it is deferring schema construction.