[ty] Support as-patterns in reachability analysis

[sharkdp]

Summary

Support as patterns in reachability analysis:

from typing import assert_never


def f(subject: str | int):
    match subject:
        case int() as x:
            pass
        case str():
            pass
        case _:
            assert_never(subject)  # would previously emit an error

Note that we still don't support inferring correct types for the bound name (x).

Closes astral-sh/ty#928

Test Plan

New Markdown tests

[github-actions]

Diagnostic diff on typing conformance tests

Changes were detected when running ty on typing conformance tests

--- old-output.txt	2025-08-04 18:05:50.549478476 +0000
+++ new-output.txt	2025-08-04 18:05:50.611478863 +0000
@@ -532,31 +532,23 @@
 generics_type_erasure.py:40:16: error[invalid-argument-type] Argument to bound method `__init__` is incorrect: Expected `str | None`, found `Literal[0]`
 generics_type_erasure.py:47:1: error[type-assertion-failure] Argument does not have asserted type `int`
 generics_type_erasure.py:56:1: error[type-assertion-failure] Argument does not have asserted type `bytes`
-generics_typevartuple_args.py:16:34: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_args.py:20:1: error[type-assertion-failure] Argument does not have asserted type `tuple[int, str]`
-generics_typevartuple_args.py:27:77: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_args.py:31:1: error[type-assertion-failure] Argument does not have asserted type `tuple[()]`
 generics_typevartuple_args.py:32:1: error[type-assertion-failure] Argument does not have asserted type `tuple[int, str]`
 generics_typevartuple_basic.py:12:14: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
-generics_typevartuple_basic.py:16:26: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_basic.py:23:13: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
-generics_typevartuple_basic.py:42:34: error[invalid-argument-type] Argument to bound method `__init__` is incorrect: Expected `tuple[@Todo, ...]`, found `Literal[1]`
 generics_typevartuple_basic.py:52:14: error[invalid-argument-type] `TypeVarTuple` is not a valid argument to `Generic`
 generics_typevartuple_basic.py:65:27: error[unknown-argument] Argument `covariant` does not match any known parameter of function `__new__`
 generics_typevartuple_basic.py:66:27: error[too-many-positional-arguments] Too many positional arguments to function `__new__`: expected 2, got 4
 generics_typevartuple_basic.py:67:27: error[unknown-argument] Argument `bound` does not match any known parameter of function `__new__`
-generics_typevartuple_basic.py:75:50: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_basic.py:84:1: error[type-assertion-failure] Argument does not have asserted type `tuple[int]`
 generics_typevartuple_basic.py:106:14: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
-generics_typevartuple_callable.py:29:57: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_callable.py:33:54: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[int | float | complex, str, int]`
 generics_typevartuple_callable.py:37:34: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[str]`
 generics_typevartuple_callable.py:41:1: error[type-assertion-failure] Argument does not have asserted type `tuple[str, int, int | float | complex]`
 generics_typevartuple_callable.py:42:1: error[type-assertion-failure] Argument does not have asserted type `tuple[str]`
-generics_typevartuple_callable.py:45:43: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_callable.py:49:1: error[type-assertion-failure] Argument does not have asserted type `tuple[int | float, str, int | float | complex]`
 generics_typevartuple_concat.py:22:13: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
-generics_typevartuple_concat.py:47:42: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, ...]`
 generics_typevartuple_concat.py:52:1: error[type-assertion-failure] Argument does not have asserted type `tuple[int, bool, str]`
 generics_typevartuple_overloads.py:16:13: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
 generics_typevartuple_specialization.py:16:13: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
@@ -568,14 +560,15 @@
 generics_typevartuple_specialization.py:52:5: error[type-assertion-failure] Argument does not have asserted type `tuple[str, @Todo]`
 generics_typevartuple_specialization.py:52:37: error[invalid-type-form] Tuple literals are not allowed in this context in a type expression: Did you mean `tuple[()]`?
 generics_typevartuple_specialization.py:59:14: error[invalid-argument-type] `@Todo` is not a valid argument to `Generic`
+generics_typevartuple_specialization.py:92:67: error[invalid-type-form] Variable of type `type[@Todo]` is not allowed in a type expression
 generics_typevartuple_specialization.py:93:5: error[type-assertion-failure] Argument does not have asserted type `tuple[str, int]`
 generics_typevartuple_specialization.py:94:5: error[type-assertion-failure] Argument does not have asserted type `tuple[int | float]`
 generics_typevartuple_specialization.py:95:5: error[type-assertion-failure] Argument does not have asserted type `tuple[Any, *tuple[Any, ...]]`
-generics_typevartuple_specialization.py:130:35: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[tuple[@Todo, ...], T1@func7, T2@func7]`
+generics_typevartuple_specialization.py:130:35: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, T1@func7, T2@func7]`
 generics_typevartuple_specialization.py:135:5: error[type-assertion-failure] Argument does not have asserted type `tuple[tuple[()], str, bool]`
 generics_typevartuple_specialization.py:136:5: error[type-assertion-failure] Argument does not have asserted type `tuple[tuple[str], bool, int | float]`
 generics_typevartuple_specialization.py:137:5: error[type-assertion-failure] Argument does not have asserted type `tuple[tuple[str, bool], int | float, int]`
-generics_typevartuple_specialization.py:143:39: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[tuple[@Todo, ...], T1@func9, T2@func9, T3@func9]`
+generics_typevartuple_specialization.py:143:39: error[invalid-return-type] Function always implicitly returns `None`, which is not assignable to return type `tuple[@Todo, T1@func9, T2@func9, T3@func9]`
 generics_typevartuple_specialization.py:148:5: error[type-assertion-failure] Argument does not have asserted type `tuple[tuple[()], str, bool, int | float]`
 generics_typevartuple_specialization.py:149:5: error[type-assertion-failure] Argument does not have asserted type `tuple[tuple[bool], str, int | float, int]`
 generics_typevartuple_specialization.py:157:5: error[type-assertion-failure] Argument does not have asserted type `tuple[*tuple[int, ...], int]`
@@ -896,4 +889,4 @@
 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 897 diagnostics
+Found 890 diagnostics

[sharkdp]

I changed the order here (and matched on I(…) instead of object(…)). The test would fail otherwise because we would detect the P | Q pattern to be always reachable (and the ones behind it to not be reachable). I don't think this was the original intention of this test (or the assertion below should have a TODO).

[AlexWaygood]

yup, the intent of the test is just to check that the re_exports query correctly identifies both Or patterns and Class patterns as binding names in the global scope

[github-actions]

mypy_primer results

No ecosystem changes detected ✅
No memory usage changes detected ✅

[github-actions]

ecosystem-analyzer results

No changes detected ✅
Full report with detailed diff