dict.setdefault on an unpinned dict always makes the dict nullable (overload evaluation premature pinning)

[yangdanny97]

Describe the Bug

available_clients = {}
def process_device(source, device):
    available_clients.setdefault(device.machineIdentifier, {"device": device}) # makes the value type dict | None
    available_clients[device.machineIdentifier].setdefault("source", source) # error!

The standard library type annotation on MutableMapping.setdefault weird here

class MutableMapping:
    # This overload should be allowed only if the value type is compatible with None.
    #
    # Keep the following methods in line with MutableMapping.setdefault, modulo positional-only differences:
    # -- collections.OrderedDict.setdefault
    # -- collections.ChainMap.setdefault
    # -- weakref.WeakKeyDictionary.setdefault
    @overload
    def setdefault(self: MutableMapping[_KT, _T | None], key: _KT, default: None = None, /) -> _T | None: ...
    @overload
    def setdefault(self, key: _KT, default: _VT, /) -> _VT: ...

I think maybe overload evaluation is pinning the type too early based on the first overload, even when it was not selected.

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIqAbqtKtrAPoDGUEM6ALnAAQC8PYAF8AOlhhgexAE65m8OI0wwqEeQAo4uAK7T5AGh7LV8gJSIxPKz2q0o9Jq3Zc4hODE7KwqbVE7rjNRhCAFtUZgALCHQYAEllLghIGGlDYBEQQPkMxCMVIKFTS2tbOgYYFjYObgBtLOCwyOi4hM4k9mkAXTcPLx8-dQytXWyQQ2G9GFMxkDJpCShSQk5cEKgKAGIeAAVSebBFnjQsPHweZlx0SABzXVQ2y8IxLYBlGBgeCM5OYjhEAHp-nMFktcNJrv8OP9MHI4P8LlcILdpPcIJd-jwwGCbDQyrBzpcbncHugeLhiCTXGIyJwIpcALRUFJwNGkgQZADMhAAjAAmDJiEBCfSUZhtJkAMWgMAoxxwBBI5CEQA

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

This is #2470