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Dict-literal arguments don't try all members of a union of dict[...] types #4913
Description
Describe the Bug
from typing import Generic, Hashable, TypeVar, reveal_type
S1 = TypeVar("S1")
K = TypeVar("K", bound=Hashable)
class Box(Generic[S1]):
def __init__(self, data: dict[str, list[int]] | dict[K, S1]) -> None: ...
reveal_type(Box({"a": 1, "b": 2}))ERROR sandbox.py:9:23-24: `Literal[1]` is not assignable to dict value type `list[int]` [[bad-assignment](https://pyrefly.org/en/docs/error-kinds/#bad-assignment)]
ERROR sandbox.py:9:31-32: `Literal[2]` is not assignable to dict value type `list[int]` [[bad-assignment](https://pyrefly.org/en/docs/error-kinds/#bad-assignment)]I think this should pass the dict[K, S1] part?
If we write it as
d: dict[str, int] = {"a": 1, "b": 2}
reveal_type(Box(d)) # same dict, via a pre-typed variablethen Pyrefly accepts it
Spotted in pandas-stubs
Sandbox Link
(Only applicable for extension issues) IDE Information
No response
Reacted by Yi-Fan Wang
Activity
Divyansh151005 commentedon Oct 28, 2026
#claim
I'd like to work on this. I traced it to dict-literal hint decomposition:
dict[K, S1]); the bound onKdoesn't matter.dict[K, int]anddict[str, S1]already work, and an unboundeddict[str, list[int]] | dict[KU, S1]fails the same way.dict_items_infer(and theDictComppath), the decompose closure returnsNonewhen both the key and value hints are unresolved.infer_with_decomposed_hinttreatsNoneas "this member isn't a dict", sodict[K, S1]is skipped before inference runs. We then fall back todict[str, list[int]], which matched with errors.Plan: have
decompose_dictreturnNoneonly when the hint can't be a dict. A dict hint with placeholder vars should still be decomposed even when the key and value hints are unresolved, since inferring against it is what pins those vars. Uninformative var-free members such asobjectkeep being skipped, so a more informative sibling likedict[str, float]still wins. I'll add regression tests for the issue example (both union orders), the dict-comprehension path, and the unchanged cases.github-actions commentedon Oct 28, 2026
@Divyansh151005 you've claimed this issue — it's now assigned to you. Thanks for picking it up! 🎉