Store the type for assignment expr (walrus) targets (#9479)

Fixes #9054

Author: llchan

mypy/checkexpr.py

@@ -3788,6 +3788,9 @@ def accept(self,
         assert typ is not None
         self.chk.store_type(node, typ)
 
+        if isinstance(node, AssignmentExpr) and not has_uninhabited_component(typ):
+            self.chk.store_type(node.target, typ)
+
         if (self.chk.options.disallow_any_expr and
                 not always_allow_any and
                 not self.chk.is_stub and

test-data/unit/check-python38.test

@@ -377,3 +377,13 @@ def check_partial_list() -> None:
         z.append(3)
     reveal_type(z)  # N: Revealed type is 'builtins.list[builtins.int]'
 [builtins fixtures/list.pyi]
+
+[case testWalrusExpr]
+def func() -> None:
+    foo = Foo()
+    if x := foo.x:
+        pass
+
+class Foo:
+    def __init__(self) -> None:
+        self.x = 123