[ty] Disallow typing.TypedDict in type expressions

[sharkdp]

Summary

Disallow typing.TypedDict in type expressions.

Related reference: python/mypy#11030

Test Plan

New Markdown tests, checked ecosystem and conformance test impact.

[github-actions]

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-08-06 13:45:47.198830453 +0000
+++ new-output.txt	2025-08-06 13:45:47.262830278 +0000
@@ -885,4 +885,5 @@
 tuples_type_form.py:36:1: error[invalid-assignment] Object of type `tuple[Literal[1], Literal[2], Literal[3], Literal[""]]` is not assignable to `tuple[int, ...]`
 typeddicts_operations.py:60:1: error[type-assertion-failure] Argument does not have asserted type `str | None`
 typeddicts_type_consistency.py:101:1: error[invalid-assignment] Object of type `Unknown | None` is not assignable to `str`
-Found 886 diagnostics
+typeddicts_usage.py:40:24: error[invalid-type-form] The special form `typing.TypedDict` is not allowed in type expressions. Did you mean to use a concrete TypedDict or `collections.abc.Mapping[str, object]` instead?
+Found 887 diagnostics

[github-actions]

mypy_primer results

Changes were detected when running on open source projects

pydantic (https://github.com/pydantic/pydantic)
- pydantic/v1/annotated_types.py:13:73: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- pydantic/v1/annotated_types.py:24:40: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- pydantic/v1/validators.py:621:67: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- pydantic/v1/validators.py:632:70: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
- Found 772 diagnostics
+ Found 768 diagnostics

No memory usage changes detected ✅

[AlexWaygood]

nit (and it's fine to leave this for a followup): I wonder if we could be a bit more helpful with this error message -- naively this might feel like a sensible thing to do if you're new to typing? Maybe something like this:

Suggested change

	x: TypedDict = {"name": "Alice"} # error: [invalid-type-form] "`typing.TypedDict` is not allowed in type expressions"
	x: TypedDict = {"name": "Alice"} # error: [invalid-type-form] "Function `typing.TypedDict` is not allowed in type expressions"

with a subdiagnostic "note: Consider using collections.abc.Mapping[str, object]"?

[sharkdp]

I quickly looked into it but it's not straightforward to add subdiagnostics/hints at this position.

Concerning "Function typing.TypedDict": I know that it's a function at runtime, but if you're unfamiliar with Python typing, then you might be even more confused by this? "I wanted to use a type, not a function…".

Even the Python documentation doesn't reveal that it's a function.

Leaving the diagnostic as-is for now, but also not opposed to a change.

[AlexWaygood]

Concerning "Function typing.TypedDict": I know that it's a function at runtime, but if you're unfamiliar with Python typing, then you might be even more confused by this? "I wanted to use a type, not a function…".

Yeah — the main thing I think might be nice to clarify is that obviously TypedDict types are allowed in type expressions; it's just the TypedDict symbol itself that is not really a type, and is thus disallowed in type expressions.

Anyway, fine to leave as-is for now!

[sharkdp]

That makes sense. I made an attempt to improve it. With a long message instead of a .info for now.

[AlexWaygood]

Thank you, the revised message is great!