[ty] Support narrowing for extended walrus targets

crates/ty_python_semantic/resources/mdtest/assignment/augmented.md

@@ -16,6 +16,15 @@ x += (3, 4)
 reveal_type(x)  # revealed: tuple[Literal[1, 2, 3, 4], ...]
 ```
 
+## Walrus target
+
+```py
+def f(xs: list[int | str]) -> None:
+    ys = xs
+    ys[0] = "s"
+    (ys := [1])[0] += 1
+```
+
 ## Dunder methods
 
 ```py

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -2504,6 +2504,20 @@ class C:
 C().x
 ```
 
+### Walrus reassignment of `self`
+
+```py
+class Other:
+    x: int = 1
+
+class C:
+    def __init__(self, other: Other) -> None:
+        (self := other).x = 1
+
+# error: [unresolved-attribute]
+reveal_type(C(Other()).x)  # revealed: Unknown
+```
+
 ### Assignment to `self` after nested function
 
 ```py

crates/ty_python_semantic/resources/mdtest/expression/attribute.md

@@ -32,3 +32,18 @@ def _(flag: bool):
     # error: [unresolved-attribute] "Class `A` has no attribute `non_existent`"
     reveal_type(A.non_existent)  # revealed: Unknown
 ```
+
+## Walrus attribute access after later rebinding
+
+```py
+class IntBox:
+    attr: int
+
+class StrBox:
+    attr: str
+
+def f() -> None:
+    (box := IntBox()).attr = 1
+    box = StrBox()
+    reveal_type(box.attr)  # revealed: str
+```

crates/ty_python_semantic/resources/mdtest/narrow/truthiness.md

@@ -46,6 +46,27 @@ else:
     reveal_type(x)  # revealed: Literal[b"", b"bar", 0, False, ""] | None | tuple[()]
 ```
 
+## Walrus Member Access
+
+We can narrow on an attribute expression, even when its base is a named expression:
+
+```py
+class Foo:
+    val: int | None
+
+if (foo := Foo()).val:
+    reveal_type(foo.val)  # revealed: int & ~AlwaysFalsy
+```
+
+But we don't pick up stale narrowings from before the assignment in the named expression:
+
+```py
+foo1 = Foo()
+foo1.val = None
+if (foo1 := Foo()).val:
+    reveal_type(foo1.val)  # revealed: int & ~AlwaysFalsy
+```
+
 ## Function Literals
 
 Basically functions are always truthy.

crates/ty_python_semantic/resources/mdtest/subscript/lists.md

@@ -33,3 +33,30 @@ x["a" if (y := 2) else 1] = 6
 # error: [invalid-assignment]
 x["a" if (y := 2) else "b"] = 6
 ```
+
+## Walrus subscript access
+
+```py
+xs: list[int | None] = [1]
+xs[0] = None
+
+reveal_type((xs := [1])[0])  # revealed: int | None
+```
+
+## Walrus subscript access after rebinding
+
+```py
+def f(xs: list[int | str]) -> None:
+    ys = xs
+    ys[0] = "s"
+    reveal_type((ys := [1])[0])  # revealed: int
+```
+
+## Walrus subscript access after later rebinding
+
+```py
+def f() -> None:
+    (ys := [1])[0] = 2
+    ys = ["s"]
+    reveal_type(ys[0])  # revealed: str
+```