[ty] Async for loops and async iterables (#19634)

## Summary

Add support for `async for` loops and async iterables.

part of https://github.com/astral-sh/ty/issues/151

## Ecosystem impact

```diff
- boostedblob/listing.py:445:54: warning[unused-ignore-comment] Unused blanket `type: ignore` directive
```

This is correct. We now find a true positive in the `# type: ignore`'d
code.

All of the other ecosystem hits are of the type

```diff
trio (https://github.com/python-trio/trio)
+ src/trio/_core/_tests/test_guest_mode.py:532:24: error[not-iterable] Object of type `MemorySendChannel[int] | MemoryReceiveChannel[int]` may not be iterable
```

The message is correct, because only `MemoryReceiveChannel` has an
`__aiter__` method, but `MemorySendChannel` does not. What's not correct
is our inferred type here. It should be `MemoryReceiveChannel[int]`, not
the union of the two. This is due to missing unpacking support for tuple
subclasses, which @AlexWaygood is working on. I don't think this should
block merging this PR, because those wrong types are already there,
without this PR.

## Test Plan

New Markdown tests and snapshot tests for diagnostics.

crates/ty_python_semantic/resources/mdtest/comprehensions/basic.md

@@ -128,7 +128,7 @@ class AsyncIterable:
         return AsyncIterator()
 
 async def _():
-    # revealed: @Todo(async iterables/iterators)
+    # revealed: int
     [reveal_type(x) async for x in AsyncIterable()]
 ```
 
@@ -147,6 +147,7 @@ class Iterable:
         return Iterator()
 
 async def _():
-    # revealed: @Todo(async iterables/iterators)
+    # error: [not-iterable] "Object of type `Iterable` is not async-iterable"
+    # revealed: Unknown
     [reveal_type(x) async for x in Iterable()]
 ```

crates/ty_python_semantic/resources/mdtest/diagnostics/semantic_syntax_errors.md

@@ -27,6 +27,7 @@ If all of the comprehensions are `async`, on the other hand, the code was still
 
 ```py
 async def test():
+    # error: [not-iterable] "Object of type `range` is not async-iterable"
     return [[x async for x in elements(n)] async for n in range(3)]
 ```
 

crates/ty_python_semantic/resources/mdtest/loops/async_for.md

@@ -2,27 +2,6 @@
 
 Async `for` loops do not work according to the synchronous iteration protocol.
 
-## Invalid async for loop
-
-```py
-async def foo():
-    class Iterator:
-        def __next__(self) -> int:
-            return 42
-
-    class Iterable:
-        def __iter__(self) -> Iterator:
-            return Iterator()
-
-    async for x in Iterator():
-        pass
-
-    # TODO: should reveal `Unknown` because `__aiter__` is not defined
-    # revealed: @Todo(async iterables/iterators)
-    # error: [possibly-unresolved-reference]
-    reveal_type(x)
-```
-
 ## Basic async for loop
 
 ```py
@@ -35,11 +14,154 @@ async def foo():
         def __aiter__(self) -> IntAsyncIterator:
             return IntAsyncIterator()
 
-    # TODO(Alex): async iterables/iterators!
     async for x in IntAsyncIterable():
-        pass
-
-    # error: [possibly-unresolved-reference]
-    # revealed: @Todo(async iterables/iterators)
-    reveal_type(x)
+        reveal_type(x)  # revealed: int
+```
+
+## Async for loop with unpacking
+
+```py
+async def foo():
+    class AsyncIterator:
+        async def __anext__(self) -> tuple[int, str]:
+            return 42, "hello"
+
+    class AsyncIterable:
+        def __aiter__(self) -> AsyncIterator:
+            return AsyncIterator()
+
+    async for x, y in AsyncIterable():
+        reveal_type(x)  # revealed: int
+        reveal_type(y)  # revealed: str
+```
+
+## Error cases
+
+<!-- snapshot-diagnostics -->
+
+### No `__aiter__` method
+
+```py
+from typing_extensions import reveal_type
+
+class NotAsyncIterable: ...
+
+async def foo():
+    # error: [not-iterable] "Object of type `NotAsyncIterable` is not async-iterable"
+    async for x in NotAsyncIterable():
+        reveal_type(x)  # revealed: Unknown
+```
+
+### Synchronously iterable, but not asynchronously iterable
+
+```py
+from typing_extensions import reveal_type
+
+async def foo():
+    class Iterator:
+        def __next__(self) -> int:
+            return 42
+
+    class Iterable:
+        def __iter__(self) -> Iterator:
+            return Iterator()
+
+    # error: [not-iterable] "Object of type `Iterator` is not async-iterable"
+    async for x in Iterator():
+        reveal_type(x)  # revealed: Unknown
+```
+
+### No `__anext__` method
+
+```py
+from typing_extensions import reveal_type
+
+class NoAnext: ...
+
+class AsyncIterable:
+    def __aiter__(self) -> NoAnext:
+        return NoAnext()
+
+async def foo():
+    # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+    async for x in AsyncIterable():
+        reveal_type(x)  # revealed: Unknown
+```
+
+### Possibly unbound `__anext__` method
+
+```py
+from typing_extensions import reveal_type
+
+async def foo(flag: bool):
+    class PossiblyUnboundAnext:
+        if flag:
+            async def __anext__(self) -> int:
+                return 42
+
+    class AsyncIterable:
+        def __aiter__(self) -> PossiblyUnboundAnext:
+            return PossiblyUnboundAnext()
+
+    # error: [not-iterable] "Object of type `AsyncIterable` may not be async-iterable"
+    async for x in AsyncIterable():
+        reveal_type(x)  # revealed: int
+```
+
+### Possibly unbound `__aiter__` method
+
+```py
+from typing_extensions import reveal_type
+
+async def foo(flag: bool):
+    class AsyncIterable:
+        async def __anext__(self) -> int:
+            return 42
+
+    class PossiblyUnboundAiter:
+        if flag:
+            def __aiter__(self) -> AsyncIterable:
+                return AsyncIterable()
+
+    # error: "Object of type `PossiblyUnboundAiter` may not be async-iterable"
+    async for x in PossiblyUnboundAiter():
+        reveal_type(x)  # revealed: int
+```
+
+### Wrong signature for `__aiter__`
+
+```py
+from typing_extensions import reveal_type
+
+class AsyncIterator:
+    async def __anext__(self) -> int:
+        return 42
+
+class AsyncIterable:
+    def __aiter__(self, arg: int) -> AsyncIterator:  # wrong
+        return AsyncIterator()
+
+async def foo():
+    # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+    async for x in AsyncIterable():
+        reveal_type(x)  # revealed: int
+```
+
+### Wrong signature for `__anext__`
+
+```py
+from typing_extensions import reveal_type
+
+class AsyncIterator:
+    async def __anext__(self, arg: int) -> int:  # wrong
+        return 42
+
+class AsyncIterable:
+    def __aiter__(self) -> AsyncIterator:
+        return AsyncIterator()
+
+async def foo():
+    # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+    async for x in AsyncIterable():
+        reveal_type(x)  # revealed: int
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_No_`__aiter__`_metho…_(4fbd80e21774cc23).snap

@@ -0,0 +1,52 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - No `__aiter__` method
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from typing_extensions import reveal_type
+2 | 
+3 | class NotAsyncIterable: ...
+4 | 
+5 | async def foo():
+6 |     # error: [not-iterable] "Object of type `NotAsyncIterable` is not async-iterable"
+7 |     async for x in NotAsyncIterable():
+8 |         reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `NotAsyncIterable` is not async-iterable
+ --> src/mdtest_snippet.py:7:20
+  |
+5 | async def foo():
+6 |     # error: [not-iterable] "Object of type `NotAsyncIterable` is not async-iterable"
+7 |     async for x in NotAsyncIterable():
+  |                    ^^^^^^^^^^^^^^^^^^
+8 |         reveal_type(x)  # revealed: Unknown
+  |
+info: It has no `__aiter__` method
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+ --> src/mdtest_snippet.py:8:21
+  |
+6 |     # error: [not-iterable] "Object of type `NotAsyncIterable` is not async-iterable"
+7 |     async for x in NotAsyncIterable():
+8 |         reveal_type(x)  # revealed: Unknown
+  |                     ^ `Unknown`
+  |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_No_`__anext__`_metho…_(a0b186714127abee).snap

@@ -0,0 +1,56 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - No `__anext__` method
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class NoAnext: ...
+ 4 | 
+ 5 | class AsyncIterable:
+ 6 |     def __aiter__(self) -> NoAnext:
+ 7 |         return NoAnext()
+ 8 | 
+ 9 | async def foo():
+10 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+11 |     async for x in AsyncIterable():
+12 |         reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `AsyncIterable` is not async-iterable
+  --> src/mdtest_snippet.py:11:20
+   |
+ 9 | async def foo():
+10 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+11 |     async for x in AsyncIterable():
+   |                    ^^^^^^^^^^^^^^^
+12 |         reveal_type(x)  # revealed: Unknown
+   |
+info: Its `__aiter__` method returns an object of type `NoAnext`, which has no `__anext__` method
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:12:21
+   |
+10 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+11 |     async for x in AsyncIterable():
+12 |         reveal_type(x)  # revealed: Unknown
+   |                     ^ `Unknown`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_Possibly_unbound_`__…_(42b1d61a2b7be1b5).snap

@@ -0,0 +1,58 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - Possibly unbound `__aiter__` method
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | async def foo(flag: bool):
+ 4 |     class AsyncIterable:
+ 5 |         async def __anext__(self) -> int:
+ 6 |             return 42
+ 7 | 
+ 8 |     class PossiblyUnboundAiter:
+ 9 |         if flag:
+10 |             def __aiter__(self) -> AsyncIterable:
+11 |                 return AsyncIterable()
+12 | 
+13 |     # error: "Object of type `PossiblyUnboundAiter` may not be async-iterable"
+14 |     async for x in PossiblyUnboundAiter():
+15 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `PossiblyUnboundAiter` may not be async-iterable
+  --> src/mdtest_snippet.py:14:20
+   |
+13 |     # error: "Object of type `PossiblyUnboundAiter` may not be async-iterable"
+14 |     async for x in PossiblyUnboundAiter():
+   |                    ^^^^^^^^^^^^^^^^^^^^^^
+15 |         reveal_type(x)  # revealed: int
+   |
+info: Its `__aiter__` attribute (with type `bound method PossiblyUnboundAiter.__aiter__() -> AsyncIterable`) may not be callable
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:15:21
+   |
+13 |     # error: "Object of type `PossiblyUnboundAiter` may not be async-iterable"
+14 |     async for x in PossiblyUnboundAiter():
+15 |         reveal_type(x)  # revealed: int
+   |                     ^ `int`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_Possibly_unbound_`__…_(74ad2f945cad6ed8).snap

@@ -0,0 +1,58 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - Possibly unbound `__anext__` method
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | async def foo(flag: bool):
+ 4 |     class PossiblyUnboundAnext:
+ 5 |         if flag:
+ 6 |             async def __anext__(self) -> int:
+ 7 |                 return 42
+ 8 | 
+ 9 |     class AsyncIterable:
+10 |         def __aiter__(self) -> PossiblyUnboundAnext:
+11 |             return PossiblyUnboundAnext()
+12 | 
+13 |     # error: [not-iterable] "Object of type `AsyncIterable` may not be async-iterable"
+14 |     async for x in AsyncIterable():
+15 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `AsyncIterable` may not be async-iterable
+  --> src/mdtest_snippet.py:14:20
+   |
+13 |     # error: [not-iterable] "Object of type `AsyncIterable` may not be async-iterable"
+14 |     async for x in AsyncIterable():
+   |                    ^^^^^^^^^^^^^^^
+15 |         reveal_type(x)  # revealed: int
+   |
+info: Its `__aiter__` method returns an object of type `PossiblyUnboundAnext`, which may not have a `__anext__` method
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:15:21
+   |
+13 |     # error: [not-iterable] "Object of type `AsyncIterable` may not be async-iterable"
+14 |     async for x in AsyncIterable():
+15 |         reveal_type(x)  # revealed: int
+   |                     ^ `int`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_Synchronously_iterab…_(80fa705b1c61d982).snap

@@ -0,0 +1,57 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - Synchronously iterable, but not asynchronously iterable
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | async def foo():
+ 4 |     class Iterator:
+ 5 |         def __next__(self) -> int:
+ 6 |             return 42
+ 7 | 
+ 8 |     class Iterable:
+ 9 |         def __iter__(self) -> Iterator:
+10 |             return Iterator()
+11 | 
+12 |     # error: [not-iterable] "Object of type `Iterator` is not async-iterable"
+13 |     async for x in Iterator():
+14 |         reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `Iterator` is not async-iterable
+  --> src/mdtest_snippet.py:13:20
+   |
+12 |     # error: [not-iterable] "Object of type `Iterator` is not async-iterable"
+13 |     async for x in Iterator():
+   |                    ^^^^^^^^^^
+14 |         reveal_type(x)  # revealed: Unknown
+   |
+info: It has no `__aiter__` method
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:14:21
+   |
+12 |     # error: [not-iterable] "Object of type `Iterator` is not async-iterable"
+13 |     async for x in Iterator():
+14 |         reveal_type(x)  # revealed: Unknown
+   |                     ^ `Unknown`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_Wrong_signature_for_…_(b614724363eec343).snap

@@ -0,0 +1,59 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - Wrong signature for `__anext__`
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class AsyncIterator:
+ 4 |     async def __anext__(self, arg: int) -> int:  # wrong
+ 5 |         return 42
+ 6 | 
+ 7 | class AsyncIterable:
+ 8 |     def __aiter__(self) -> AsyncIterator:
+ 9 |         return AsyncIterator()
+10 | 
+11 | async def foo():
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+14 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `AsyncIterable` is not async-iterable
+  --> src/mdtest_snippet.py:13:20
+   |
+11 | async def foo():
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+   |                    ^^^^^^^^^^^^^^^
+14 |         reveal_type(x)  # revealed: int
+   |
+info: Its `__aiter__` method returns an object of type `AsyncIterator`, which has an invalid `__anext__` method
+info: Expected signature for `__anext__` is `def __anext__(self): ...`
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:14:21
+   |
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+14 |         reveal_type(x)  # revealed: int
+   |                     ^ `int`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/async_for.md_-_Async_-_Error_cases_-_Wrong_signature_for_…_(e1f3e9275d0a367).snap

@@ -0,0 +1,59 @@
+---
+source: crates/ty_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: async_for.md - Async - Error cases - Wrong signature for `__aiter__`
+mdtest path: crates/ty_python_semantic/resources/mdtest/loops/async_for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class AsyncIterator:
+ 4 |     async def __anext__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | class AsyncIterable:
+ 8 |     def __aiter__(self, arg: int) -> AsyncIterator:  # wrong
+ 9 |         return AsyncIterator()
+10 | 
+11 | async def foo():
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+14 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error[not-iterable]: Object of type `AsyncIterable` is not async-iterable
+  --> src/mdtest_snippet.py:13:20
+   |
+11 | async def foo():
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+   |                    ^^^^^^^^^^^^^^^
+14 |         reveal_type(x)  # revealed: int
+   |
+info: Its `__aiter__` method has an invalid signature
+info: Expected signature `def __aiter__(self): ...`
+info: rule `not-iterable` is enabled by default
+
+```
+
+```
+info[revealed-type]: Revealed type
+  --> src/mdtest_snippet.py:14:21
+   |
+12 |     # error: [not-iterable] "Object of type `AsyncIterable` is not async-iterable"
+13 |     async for x in AsyncIterable():
+14 |         reveal_type(x)  # revealed: int
+   |                     ^ `int`
+   |
+
+```

crates/ty_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_invalid_`__…_(c626bde8651b643a).snap

@@ -60,7 +60,7 @@ error[not-iterable]: Object of type `Iterable1` may not be iterable
 29 |         reveal_type(x)  # revealed: int | str
    |
 info: Its `__iter__` method returns an object of type `Iterator1`, which may have an invalid `__next__` method
-info: Expected signature for `__next__` is `def __next__(self): ...`)
+info: Expected signature for `__next__` is `def __next__(self): ...`
 info: rule `not-iterable` is enabled by default
 
 ```

crates/ty_python_semantic/resources/mdtest/snapshots/semantic_syntax_erro…_-_Semantic_syntax_erro…_-_`async`_comprehensio…_-_Python_3.10_(96aa8ec77d46553d).snap

@@ -19,14 +19,15 @@ mdtest path: crates/ty_python_semantic/resources/mdtest/diagnostics/semantic_syn
  5 |     # error: 19 [invalid-syntax] "cannot use an asynchronous comprehension inside of a synchronous comprehension on Python 3.10 (syntax was added in 3.11)"
  6 |     return {n: [x async for x in elements(n)] for n in range(3)}
  7 | async def test():
- 8 |     return [[x async for x in elements(n)] async for n in range(3)]
- 9 | async def f():
-10 |     [x for x in [1]] and [x async for x in elements(1)]
-11 | 
-12 | async def f():
-13 |     def g():
-14 |         pass
-15 |     [x async for x in elements(1)]
+ 8 |     # error: [not-iterable] "Object of type `range` is not async-iterable"
+ 9 |     return [[x async for x in elements(n)] async for n in range(3)]
+10 | async def f():
+11 |     [x for x in [1]] and [x async for x in elements(1)]
+12 | 
+13 | async def f():
+14 |     def g():
+15 |         pass
+16 |     [x async for x in elements(1)]
 ```
 
 # Diagnostics
@@ -40,7 +41,23 @@ error[invalid-syntax]
 6 |     return {n: [x async for x in elements(n)] for n in range(3)}
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use an asynchronous comprehension inside of a synchronous comprehension on Python 3.10 (syntax was added in 3.11)
 7 | async def test():
-8 |     return [[x async for x in elements(n)] async for n in range(3)]
+8 |     # error: [not-iterable] "Object of type `range` is not async-iterable"
   |
 
 ```
+
+```
+error[not-iterable]: Object of type `range` is not async-iterable
+  --> src/mdtest_snippet.py:9:59
+   |
+ 7 | async def test():
+ 8 |     # error: [not-iterable] "Object of type `range` is not async-iterable"
+ 9 |     return [[x async for x in elements(n)] async for n in range(3)]
+   |                                                           ^^^^^^^^
+10 | async def f():
+11 |     [x for x in [1]] and [x async for x in elements(1)]
+   |
+info: It has no `__aiter__` method
+info: rule `not-iterable` is enabled by default
+
+```

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -49,7 +49,7 @@ use crate::semantic_index::use_def::{
 };
 use crate::semantic_index::{ArcUseDefMap, ExpressionsScopeMap, SemanticIndex};
 use crate::semantic_model::HasTrackedScope;
-use crate::unpack::{Unpack, UnpackKind, UnpackPosition, UnpackValue};
+use crate::unpack::{EvaluationMode, Unpack, UnpackKind, UnpackPosition, UnpackValue};
 use crate::{Db, Program};
 
 mod except_handlers;
@@ -2804,9 +2804,17 @@ impl<'ast> Unpackable<'ast> {
     const fn kind(&self) -> UnpackKind {
         match self {
             Unpackable::Assign(_) => UnpackKind::Assign,
-            Unpackable::For(_) | Unpackable::Comprehension { .. } => UnpackKind::Iterable,
+            Unpackable::For(ast::StmtFor { is_async, .. }) => UnpackKind::Iterable {
+                mode: EvaluationMode::from_is_async(*is_async),
+            },
+            Unpackable::Comprehension {
+                node: ast::Comprehension { is_async, .. },
+                ..
+            } => UnpackKind::Iterable {
+                mode: EvaluationMode::from_is_async(*is_async),
+            },
             Unpackable::WithItem { is_async, .. } => UnpackKind::ContextManager {
-                is_async: *is_async,
+                mode: EvaluationMode::from_is_async(*is_async),
             },
         }
     }

crates/ty_python_semantic/src/types.rs

@@ -59,6 +59,7 @@ use crate::types::mro::{Mro, MroError, MroIterator};
 pub(crate) use crate::types::narrow::infer_narrowing_constraint;
 use crate::types::signatures::{Parameter, ParameterForm, Parameters, walk_signature};
 use crate::types::tuple::{TupleSpec, TupleType};
+use crate::unpack::EvaluationMode;
 pub use crate::util::diagnostics::add_inferred_python_version_hint_to_diagnostic;
 use crate::{Db, FxOrderSet, Module, Program};
 pub(crate) use class::{ClassLiteral, ClassType, GenericAlias, KnownClass};
@@ -4637,6 +4638,65 @@ impl<'db> Type<'db> {
     /// y(*x)
     /// ```
     fn try_iterate(self, db: &'db dyn Db) -> Result<Cow<'db, TupleSpec<'db>>, IterationError<'db>> {
+        self.try_iterate_with_mode(db, EvaluationMode::Sync)
+    }
+
+    fn try_iterate_with_mode(
+        self,
+        db: &'db dyn Db,
+        mode: EvaluationMode,
+    ) -> Result<Cow<'db, TupleSpec<'db>>, IterationError<'db>> {
+        if mode.is_async() {
+            let try_call_dunder_anext_on_iterator = |iterator: Type<'db>| {
+                iterator
+                    .try_call_dunder(db, "__anext__", CallArguments::none())
+                    .map(|dunder_anext_outcome| {
+                        dunder_anext_outcome.return_type(db).resolve_await(db)
+                    })
+            };
+
+            return match self.try_call_dunder(db, "__aiter__", CallArguments::none()) {
+                Ok(dunder_aiter_bindings) => {
+                    let iterator = dunder_aiter_bindings.return_type(db);
+                    match try_call_dunder_anext_on_iterator(iterator) {
+                        Ok(result) => Ok(Cow::Owned(TupleSpec::homogeneous(result))),
+                        Err(dunder_anext_error) => {
+                            Err(IterationError::IterReturnsInvalidIterator {
+                                iterator,
+                                dunder_error: dunder_anext_error,
+                                mode,
+                            })
+                        }
+                    }
+                }
+                Err(CallDunderError::PossiblyUnbound(dunder_aiter_bindings)) => {
+                    let iterator = dunder_aiter_bindings.return_type(db);
+                    match try_call_dunder_anext_on_iterator(iterator) {
+                        Ok(_) => Err(IterationError::IterCallError {
+                            kind: CallErrorKind::PossiblyNotCallable,
+                            bindings: dunder_aiter_bindings,
+                            mode,
+                        }),
+                        Err(dunder_anext_error) => {
+                            Err(IterationError::IterReturnsInvalidIterator {
+                                iterator,
+                                dunder_error: dunder_anext_error,
+                                mode,
+                            })
+                        }
+                    }
+                }
+                Err(CallDunderError::CallError(kind, bindings)) => {
+                    Err(IterationError::IterCallError {
+                        kind,
+                        bindings,
+                        mode,
+                    })
+                }
+                Err(CallDunderError::MethodNotAvailable) => Err(IterationError::UnboundAiterError),
+            };
+        }
+
         match self {
             Type::Tuple(tuple_type) => return Ok(Cow::Borrowed(tuple_type.tuple(db))),
             Type::GenericAlias(alias) if alias.origin(db).is_tuple(db) => {
@@ -4693,7 +4753,8 @@ impl<'db> Type<'db> {
                     .map_err(
                         |dunder_next_error| IterationError::IterReturnsInvalidIterator {
                             iterator,
-                            dunder_next_error,
+                            dunder_error: dunder_next_error,
+                            mode,
                         },
                     )
             }
@@ -4728,15 +4789,18 @@ impl<'db> Type<'db> {
 
                     Err(dunder_next_error) => Err(IterationError::IterReturnsInvalidIterator {
                         iterator,
-                        dunder_next_error,
+                        dunder_error: dunder_next_error,
+                        mode,
                     }),
                 }
             }
 
             // `__iter__` is definitely bound but it can't be called with the expected arguments
-            Err(CallDunderError::CallError(kind, bindings)) => {
-                Err(IterationError::IterCallError(kind, bindings))
-            }
+            Err(CallDunderError::CallError(kind, bindings)) => Err(IterationError::IterCallError {
+                kind,
+                bindings,
+                mode,
+            }),
 
             // There's no `__iter__` method. Try `__getitem__` instead...
             Err(CallDunderError::MethodNotAvailable) => try_call_dunder_getitem()
@@ -4818,7 +4882,7 @@ impl<'db> Type<'db> {
     fn generator_return_type(self, db: &'db dyn Db) -> Option<Type<'db>> {
         // TODO: Ideally, we would first try to upcast `self` to an instance of `Generator` and *then*
         // match on the protocol instance to get the `ReturnType` type parameter. For now, implement
-        // an ad-hoc solution that works for protocols and instances of classes that directly inherit
+        // an ad-hoc solution that works for protocols and instances of classes that explicitly inherit
         // from the `Generator` protocol, such as `types.GeneratorType`.
 
         let from_class_base = |base: ClassBase<'db>| {
@@ -6806,18 +6870,24 @@ impl<'db> ContextManagerError<'db> {
 /// Error returned if a type is not (or may not be) iterable.
 #[derive(Debug)]
 enum IterationError<'db> {
-    /// The object being iterated over has a bound `__iter__` method,
+    /// The object being iterated over has a bound `__(a)iter__` method,
     /// but calling it with the expected arguments results in an error.
-    IterCallError(CallErrorKind, Box<Bindings<'db>>),
+    IterCallError {
+        kind: CallErrorKind,
+        bindings: Box<Bindings<'db>>,
+        mode: EvaluationMode,
+    },
 
-    /// The object being iterated over has a bound `__iter__` method that can be called
+    /// The object being iterated over has a bound `__(a)iter__` method that can be called
     /// with the expected types, but it returns an object that is not a valid iterator.
     IterReturnsInvalidIterator {
-        /// The type of the object returned by the `__iter__` method.
+        /// The type of the object returned by the `__(a)iter__` method.
         iterator: Type<'db>,
-        /// The error we encountered when we tried to call `__next__` on the type
-        /// returned by `__iter__`
-        dunder_next_error: CallDunderError<'db>,
+        /// The error we encountered when we tried to call `__(a)next__` on the type
+        /// returned by `__(a)iter__`
+        dunder_error: CallDunderError<'db>,
+        /// Whether this is a synchronous or an asynchronous iterator.
+        mode: EvaluationMode,
     },
 
     /// The object being iterated over has a bound `__iter__` method that returns a
@@ -6838,6 +6908,9 @@ enum IterationError<'db> {
     UnboundIterAndGetitemError {
         dunder_getitem_error: CallDunderError<'db>,
     },
+
+    /// The asynchronous iterable has no `__aiter__` method.
+    UnboundAiterError,
 }
 
 impl<'db> IterationError<'db> {
@@ -6847,16 +6920,43 @@ impl<'db> IterationError<'db> {
 
     /// Returns the element type if it is known, or `None` if the type is never iterable.
     fn element_type(&self, db: &'db dyn Db) -> Option<Type<'db>> {
+        let return_type = |result: Result<Bindings<'db>, CallDunderError<'db>>| {
+            result
+                .map(|outcome| Some(outcome.return_type(db)))
+                .unwrap_or_else(|call_error| call_error.return_type(db))
+        };
+
         match self {
             Self::IterReturnsInvalidIterator {
-                dunder_next_error, ..
-            } => dunder_next_error.return_type(db),
+                dunder_error, mode, ..
+            } => dunder_error.return_type(db).map(|ty| {
+                if mode.is_async() {
+                    ty.resolve_await(db)
+                } else {
+                    ty
+                }
+            }),
 
-            Self::IterCallError(_, dunder_iter_bindings) => dunder_iter_bindings
-                .return_type(db)
-                .try_call_dunder(db, "__next__", CallArguments::none())
-                .map(|dunder_next_outcome| Some(dunder_next_outcome.return_type(db)))
-                .unwrap_or_else(|dunder_next_call_error| dunder_next_call_error.return_type(db)),
+            Self::IterCallError {
+                kind: _,
+                bindings: dunder_iter_bindings,
+                mode,
+            } => {
+                if mode.is_async() {
+                    return_type(dunder_iter_bindings.return_type(db).try_call_dunder(
+                        db,
+                        "__anext__",
+                        CallArguments::none(),
+                    ))
+                    .map(|ty| ty.resolve_await(db))
+                } else {
+                    return_type(dunder_iter_bindings.return_type(db).try_call_dunder(
+                        db,
+                        "__next__",
+                        CallArguments::none(),
+                    ))
+                }
+            }
 
             Self::PossiblyUnboundIterAndGetitemError {
                 dunder_next_return,
@@ -6882,6 +6982,19 @@ impl<'db> IterationError<'db> {
             Self::UnboundIterAndGetitemError {
                 dunder_getitem_error,
             } => dunder_getitem_error.return_type(db),
+
+            Self::UnboundAiterError => None,
+        }
+    }
+
+    /// Does this error concern a synchronous or asynchronous iterable?
+    fn mode(&self) -> EvaluationMode {
+        match self {
+            Self::IterCallError { mode, .. } => *mode,
+            Self::IterReturnsInvalidIterator { mode, .. } => *mode,
+            Self::PossiblyUnboundIterAndGetitemError { .. }
+            | Self::UnboundIterAndGetitemError { .. } => EvaluationMode::Sync,
+            Self::UnboundAiterError => EvaluationMode::Async,
         }
     }
 
@@ -6898,6 +7011,7 @@ impl<'db> IterationError<'db> {
             db: &'a dyn Db,
             builder: LintDiagnosticGuardBuilder<'a, 'a>,
             iterable_type: Type<'a>,
+            mode: EvaluationMode,
         }
 
         impl<'a> Reporter<'a> {
@@ -6907,8 +7021,9 @@ impl<'db> IterationError<'db> {
             #[expect(clippy::wrong_self_convention)]
             fn is_not(self, because: impl std::fmt::Display) -> LintDiagnosticGuard<'a, 'a> {
                 let mut diag = self.builder.into_diagnostic(format_args!(
-                    "Object of type `{iterable_type}` is not iterable",
+                    "Object of type `{iterable_type}` is not {maybe_async}iterable",
                     iterable_type = self.iterable_type.display(self.db),
+                    maybe_async = if self.mode.is_async() { "async-" } else { "" }
                 ));
                 diag.info(because);
                 diag
@@ -6919,8 +7034,9 @@ impl<'db> IterationError<'db> {
             /// `because` should explain why `iterable_type` is likely not iterable.
             fn may_not(self, because: impl std::fmt::Display) -> LintDiagnosticGuard<'a, 'a> {
                 let mut diag = self.builder.into_diagnostic(format_args!(
-                    "Object of type `{iterable_type}` may not be iterable",
+                    "Object of type `{iterable_type}` may not be {maybe_async}iterable",
                     iterable_type = self.iterable_type.display(self.db),
+                    maybe_async = if self.mode.is_async() { "async-" } else { "" }
                 ));
                 diag.info(because);
                 diag
@@ -6931,82 +7047,107 @@ impl<'db> IterationError<'db> {
             return;
         };
         let db = context.db();
+        let mode = self.mode();
         let reporter = Reporter {
             db,
             builder,
             iterable_type,
+            mode,
         };
 
         // TODO: for all of these error variants, the "explanation" for the diagnostic
         // (everything after the "because") should really be presented as a "help:", "note",
         // or similar, rather than as part of the same sentence as the error message.
         match self {
-            Self::IterCallError(CallErrorKind::NotCallable, bindings) => {
+            Self::IterCallError {
+                kind,
+                bindings,
+                mode,
+            } => {
+                let method = if mode.is_async() {
+                    "__aiter__"
+                } else {
+                    "__iter__"
+                };
+
+                match kind {
+                    CallErrorKind::NotCallable => {
                         reporter.is_not(format_args!(
-                    "Its `__iter__` attribute has type `{dunder_iter_type}`, which is not callable",
+                        "Its `{method}` attribute has type `{dunder_iter_type}`, which is not callable",
                         dunder_iter_type = bindings.callable_type().display(db),
                     ));
                     }
-            Self::IterCallError(CallErrorKind::PossiblyNotCallable, bindings)
-                if bindings.is_single() =>
-            {
+                    CallErrorKind::PossiblyNotCallable => {
                         reporter.may_not(format_args!(
-                    "Its `__iter__` attribute (with type `{dunder_iter_type}`) \
+                            "Its `{method}` attribute (with type `{dunder_iter_type}`) \
                              may not be callable",
                             dunder_iter_type = bindings.callable_type().display(db),
                         ));
                     }
-            Self::IterCallError(CallErrorKind::PossiblyNotCallable, bindings) => {
-                reporter.may_not(format_args!(
-                    "Its `__iter__` attribute (with type `{dunder_iter_type}`) \
-                     may not be callable",
-                    dunder_iter_type = bindings.callable_type().display(db),
-                ));
-            }
-            Self::IterCallError(CallErrorKind::BindingError, bindings) if bindings.is_single() => {
+                    CallErrorKind::BindingError => {
+                        if bindings.is_single() {
                             reporter
-                    .is_not("Its `__iter__` method has an invalid signature")
-                    .info("Expected signature `def __iter__(self): ...`");
-            }
-            Self::IterCallError(CallErrorKind::BindingError, bindings) => {
-                let mut diag =
-                    reporter.may_not("Its `__iter__` method may have an invalid signature");
+                                .is_not(format_args!(
+                                    "Its `{method}` method has an invalid signature"
+                                ))
+                                .info(format_args!("Expected signature `def {method}(self): ...`"));
+                        } else {
+                            let mut diag = reporter.may_not(format_args!(
+                                "Its `{method}` method may have an invalid signature"
+                            ));
                             diag.info(format_args!(
-                    "Type of `__iter__` is `{dunder_iter_type}`",
+                                "Type of `{method}` is `{dunder_iter_type}`",
                                 dunder_iter_type = bindings.callable_type().display(db),
                             ));
-                diag.info("Expected signature for `__iter__` is `def __iter__(self): ...`");
+                            diag.info(format_args!(
+                                "Expected signature for `{method}` is `def {method}(self): ...`",
+                            ));
+                        }
+                    }
+                }
             }
 
             Self::IterReturnsInvalidIterator {
                 iterator,
-                dunder_next_error,
-            } => match dunder_next_error {
+                dunder_error: dunder_next_error,
+                mode,
+            } => {
+                let dunder_iter_name = if mode.is_async() {
+                    "__aiter__"
+                } else {
+                    "__iter__"
+                };
+                let dunder_next_name = if mode.is_async() {
+                    "__anext__"
+                } else {
+                    "__next__"
+                };
+                match dunder_next_error {
                     CallDunderError::MethodNotAvailable => {
                         reporter.is_not(format_args!(
-                        "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                     which has no `__next__` method",
+                        "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                         which has no `{dunder_next_name}` method",
                         iterator_type = iterator.display(db),
                     ));
                     }
                     CallDunderError::PossiblyUnbound(_) => {
                         reporter.may_not(format_args!(
-                        "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                     which may not have a `__next__` method",
+                            "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                            which may not have a `{dunder_next_name}` method",
                             iterator_type = iterator.display(db),
                         ));
                     }
                     CallDunderError::CallError(CallErrorKind::NotCallable, _) => {
                         reporter.is_not(format_args!(
-                        "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                         which has a `__next__` attribute that is not callable",
+                            "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                            which has a `{dunder_next_name}` attribute that is not callable",
                             iterator_type = iterator.display(db),
                         ));
                     }
                     CallDunderError::CallError(CallErrorKind::PossiblyNotCallable, _) => {
                         reporter.may_not(format_args!(
-                        "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                         which has a `__next__` attribute that may not be callable",
+                            "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                            which has a `{dunder_next_name}` attribute that may not be callable",
                             iterator_type = iterator.display(db),
                         ));
                     }
@@ -7015,22 +7156,23 @@ impl<'db> IterationError<'db> {
                     {
                         reporter
                             .is_not(format_args!(
-                            "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                             which has an invalid `__next__` method",
+                                "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                                which has an invalid `{dunder_next_name}` method",
                                 iterator_type = iterator.display(db),
                             ))
-                        .info("Expected signature for `__next__` is `def __next__(self): ...`");
+                            .info(format_args!("Expected signature for `{dunder_next_name}` is `def {dunder_next_name}(self): ...`"));
                     }
                     CallDunderError::CallError(CallErrorKind::BindingError, _) => {
                         reporter
                             .may_not(format_args!(
-                            "Its `__iter__` method returns an object of type `{iterator_type}`, \
-                             which may have an invalid `__next__` method",
+                                "Its `{dunder_iter_name}` method returns an object of type `{iterator_type}`, \
+                                which may have an invalid `{dunder_next_name}` method",
                                 iterator_type = iterator.display(db),
                             ))
-                        .info("Expected signature for `__next__` is `def __next__(self): ...`)");
+                            .info(format_args!("Expected signature for `{dunder_next_name}` is `def {dunder_next_name}(self): ...`"));
+                    }
+                }
             }
-            },
 
             Self::PossiblyUnboundIterAndGetitemError {
                 dunder_getitem_error,
@@ -7167,6 +7309,10 @@ impl<'db> IterationError<'db> {
                         );
                 }
             },
+
+            IterationError::UnboundAiterError => {
+                reporter.is_not("It has no `__aiter__` method");
+            }
         }
     }
 }

crates/ty_python_semantic/src/types/infer.rs

@@ -123,7 +123,7 @@ use crate::types::{
     TypeAndQualifiers, TypeIsType, TypeQualifiers, TypeVarBoundOrConstraints, TypeVarInstance,
     TypeVarKind, TypeVarVariance, UnionBuilder, UnionType, binding_type, todo_type,
 };
-use crate::unpack::{Unpack, UnpackPosition};
+use crate::unpack::{EvaluationMode, Unpack, UnpackPosition};
 use crate::util::diagnostics::format_enumeration;
 use crate::util::subscript::{PyIndex, PySlice};
 use crate::{Db, FxOrderSet, Program};
@@ -4560,11 +4560,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         let iterable = for_stmt.iterable(self.module());
         let target = for_stmt.target(self.module());
 
-        let loop_var_value_type = if for_stmt.is_async() {
-            let _iterable_type = self.infer_standalone_expression(iterable);
-            todo_type!("async iterables/iterators")
-        } else {
-            match for_stmt.target_kind() {
+        let loop_var_value_type = match for_stmt.target_kind() {
             TargetKind::Sequence(unpack_position, unpack) => {
                 let unpacked = infer_unpack_types(self.db(), unpack);
                 if unpack_position == UnpackPosition::First {
@@ -4575,15 +4571,18 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             }
             TargetKind::Single => {
                 let iterable_type = self.infer_standalone_expression(iterable);
+
                 iterable_type
-                        .try_iterate(self.db())
+                    .try_iterate_with_mode(
+                        self.db(),
+                        EvaluationMode::from_is_async(for_stmt.is_async()),
+                    )
                     .map(|tuple| tuple.homogeneous_element_type(self.db()))
                     .unwrap_or_else(|err| {
                         err.report_diagnostic(&self.context, iterable_type, iterable.into());
                         err.fallback_element_type(self.db())
                     })
             }
-            }
         };
 
         self.store_expression_type(target, loop_var_value_type);
@@ -5692,12 +5691,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             }
         };
 
-        let target_type = if comprehension.is_async() {
-            // TODO: async iterables/iterators! -- Alex
-            let _iterable_type = infer_iterable_type();
-            todo_type!("async iterables/iterators")
-        } else {
-            match comprehension.target_kind() {
+        let target_type = match comprehension.target_kind() {
             TargetKind::Sequence(unpack_position, unpack) => {
                 let unpacked = infer_unpack_types(self.db(), unpack);
                 if unpack_position == UnpackPosition::First {
@@ -5708,15 +5702,18 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             }
             TargetKind::Single => {
                 let iterable_type = infer_iterable_type();
+
                 iterable_type
-                        .try_iterate(self.db())
+                    .try_iterate_with_mode(
+                        self.db(),
+                        EvaluationMode::from_is_async(comprehension.is_async()),
+                    )
                     .map(|tuple| tuple.homogeneous_element_type(self.db()))
                     .unwrap_or_else(|err| {
                         err.report_diagnostic(&self.context, iterable_type, iterable.into());
                         err.fallback_element_type(self.db())
                     })
             }
-            }
         };
 
         self.expressions.insert(target.into(), target_type);

crates/ty_python_semantic/src/types/unpacker.rs

@@ -64,8 +64,8 @@ impl<'db, 'ast> Unpacker<'db, 'ast> {
                     value_type
                 }
             }
-            UnpackKind::Iterable => value_type
-                .try_iterate(self.db())
+            UnpackKind::Iterable { mode } => value_type
+                .try_iterate_with_mode(self.db(), mode)
                 .map(|tuple| tuple.homogeneous_element_type(self.db()))
                 .unwrap_or_else(|err| {
                     err.report_diagnostic(
@@ -75,8 +75,8 @@ impl<'db, 'ast> Unpacker<'db, 'ast> {
                     );
                     err.fallback_element_type(self.db())
                 }),
-            UnpackKind::ContextManager { is_async } => {
-                if is_async {
+            UnpackKind::ContextManager { mode } => {
+                if mode.is_async() {
                     value_type.aenter(self.db())
                 } else {
                     value_type.try_enter(self.db()).unwrap_or_else(|err| {

crates/ty_python_semantic/src/unpack.rs

@@ -99,12 +99,32 @@ impl<'db> UnpackValue<'db> {
     }
 }
 
+#[derive(Clone, Copy, Debug, Hash, salsa::Update)]
+pub(crate) enum EvaluationMode {
+    Sync,
+    Async,
+}
+
+impl EvaluationMode {
+    pub(crate) const fn from_is_async(is_async: bool) -> Self {
+        if is_async {
+            EvaluationMode::Async
+        } else {
+            EvaluationMode::Sync
+        }
+    }
+
+    pub(crate) const fn is_async(self) -> bool {
+        matches!(self, EvaluationMode::Async)
+    }
+}
+
 #[derive(Clone, Copy, Debug, Hash, salsa::Update)]
 pub(crate) enum UnpackKind {
     /// An iterable expression like the one in a `for` loop or a comprehension.
-    Iterable,
+    Iterable { mode: EvaluationMode },
     /// An context manager expression like the one in a `with` statement.
-    ContextManager { is_async: bool },
+    ContextManager { mode: EvaluationMode },
     /// An expression that is being assigned to a target.
     Assign,
 }