Confusing errors with anonymous TypedDict

[QEDady]

Describe the Bug

Pyrefly constructs an anonymous TypedDict for any small literal dict where all keys are literal strings, but this can lead to confusing error messages e.g.

d = {'a': 42}
d['a'] = 'b'

errors with

Literal['b'] is not assignable to TypedDict key a with type int [bad-typed-dict-key]

I think this is confusing since the user didn't specify any TypedDict. I would personally prefer a bad-assignment or bad-argument-type. Other type checkers produce one of these two, or equivalent.

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeSIAxAAQDqMUAxrgLYzUAuunAFuwAqkATjDBRS1AMoYcBADS8IcakuqpqxVAHMYAHXQ0A7nxHVSuAK7VGGaoaEQOMBXB6pTM6rHfo1eCxycpMQQ6FoqvoIcPLjohPqUCdQAEjAeHtaWxLDUYDCoHBYiyrhgispoWHj4iEkAtNQA4jCBIqhQdRwQbEHE7Ix8jADWoeF5MDiow-XUAKpw7CIAbvlQAPocwTAAFACUnNyhcH2MgaF5QiKYvfAzMPh9DmzogYaOPLzsAJIAIgCiuXyhWKvAKdnYcAsxGIuCEgW2MEIWkIagCuGYLGyLWc1C0uE6%2BMwolCjggsQUMRWQgULUYuxmtAcTjUWGoQgskVI0VimSJ1EwEBEp3EETMliE1GwQlwhgWQn0%2BjA0pYvVGKkxsMCACoFayALzUYAAclQRsQ1AALAAmAC%2B%2BkwAG0TUaALrUA1G7BGkByEBkERiUiELgsKAUGiCAMiyqyfCZdCQLRFApkuJJSQwdg8DgcYhwRAAegL-tE4kIsK0BZg6ALmHRcALzATECTQhTsQLuVhaiWqGgqGwOSbieTXR5uGIY-QcHi6DI3PQdSpcFT7uouhAAGZCABGK0b-QOtLSoRwF36DndGFwiZ1AVCrorNdGlvoWEwI36N8cOoiACOFiCreQwwKQdRTIw8DKB6hg%2BJ%2B6AgDavpTI%2BMAAGLQDAFAxtUJDkDaQA

(Only applicable for extension issues) IDE Information

No response

[NathanTempest]

Thanks for the clear report and sandbox link, @QEDady! Confirmed this reproduces on HEAD with default preset.

Comparison with other type checkers, For:

d = {'a': 42}
d['a'] = 'b'

mypy infers d: dict[str, int] and produces:

error: Incompatible types in assignment (expression has type "str", target has type "int") [assignment]

pyright infers d: dict[str, int] and produces:

error: Type "str" is not assignable to type "int"
"str" is not assignable to "int" (reportAssignmentType)

pyrefly infers an anonymous TypedDict and produces:

ERROR Literal['b'] is not assignable to TypedDict key a with type int [bad-typed-dict-key]

Neither mypy nor pyright mentions "TypedDict" because they treat the literal dict as dict[str, int] and emit a straightforward assignment-type error. Pyrefly's anonymous TypedDict inference gives more per-key precision (each key tracks its own type independently), but the error message references a concept the user never introduced.

@yangdanny97 do we change how Pyrefly emits errors here or do we change how Pyrefly infers types from Dicts. I think changing the error message to match what the user wants (emitting bad-assignment) should be the better choice.

[yangdanny97]

i have a fix coming, which does the following 3 things

1. make invalid assignments report bad-assignment rather than bad-typed-dict-key, the key exists but the value is wrong
2. print out anonymous typed dict errors differently, omitting mentions of TypedDict
3. for anonymous typed dict index errors, emit bad-index rather than bad-typed-dict-key