`implicit-any-empty-container` false positive when assigning dict containing None to `dict[str, Any]`

[Ravencentric]

Describe the Bug

Given the following code:

from typing import Any

def from_response(response: dict[str, Any]) -> None:
    response["meta"] = {"time_to_live": None}

Pyrefly reports:

❯ target\debug\pyrefly.exe --version
pyrefly 1.2.0-dev.3

❯ target\debug\pyrefly.exe check ../privatebin/t.py
ERROR Cannot infer type of empty container; it will be treated as containing `Any` [implicit-any-empty-container]
 --> C:\Users\raven\dev\github\privatebin\t.py:7:41
  |
7 |     response["meta"] = {"time_to_live": None}
  |                                         ^^^^
  |
  Consider adding a type annotation or initializing with a non-empty value
 INFO 1 error

This looks closely related to #3593, which was closed as fixed.

To check whether this was already addressed by that fix, I built Pyrefly from commit c28caa7, but I was still able to reproduce the issue there.

In this case, response is explicitly annotated as dict[str, Any], so I would expect the assigned value {"time_to_live": None} to be accepted without an implicit-any-empty-container error.

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0BBdKQA6lEejETMMMHTDUaAfUrwe6ODAAUquOs2I6mCAGMGAbTgNKAGkHCAugEo6AWgB8dAHK50MRFLodMF0uvow5iIgNDAMqFEOdAC8dMBRDFwwSgy4SlAQAG4wUYY%2BfgC%2BgSA2IGSqYFCkhDk0UBQAxHQACqT1jXRoWHj4dCa%2BkGwArpSoGb6EEp0AyjAwdAAWDAzEcIgA9Ht1so2EvGx7MOh7mLgmcHtj6BPTsxC%2Be-K8dKgFqNCo2Fgo3GECmMzmQVwxAhcAW6DIDHWvlcRUocDeQRSUQAzIQAIwAJiiEmIuliyToUSslFMDGJ6HMMEo1DRDgkk3QXB4fBgmFcxlUZkKaxSAHJQeheDBRRJJQxXKoAI6TCCqPkAaxgpFcqBMJngcApooA7qhKOgZegQOUarqMkUAGLQGAUQY4AgkcjlIA

(Only applicable for extension issues) IDE Information

No response

[yangdanny97]

I agree, this is a bug.

[yangdanny97]

Actually, now that I look more closely I'm not sure this is a bug per se but I do agree that the error isn't a useful one since the implicit-any is harmless, and flagging it is unnecessary

The fix for the other issue didn't handle this case because the only entry in the dict literal has None - without additional information, we actually are inferring an implicit-any.

It just happens that it's being assigned into a subscript of something that is fully-annotated.

So I think the correct fix would be to be smarter about when we emit these errors based on the contextual type

We can't suppress it in all subscript assignments since there are still cases where it could cause an extra Any to leak itno the code like

x = {}
x["y"] = {"z": None}

would infer x as dict[str, dict[str, Unknown | None]]

For now, the workaround would be suppressing it, or explicitly annotating that dict so the Any is explicit, like

meta: dict[str, Any | None] = {"time_to_live": None}
response["meta"] = meta