Fix partial type assignment with walrus expressions

mypy/checker.py

@@ -3539,6 +3539,18 @@ def check_assignment(
                                 var.type = None
                             del partial_types[var]
                             lvalue_type = var.type
+                        elif var.type is not None:
+                            # 右值中的 walrus 可能已解析该变量的 partial type，
+                            # 因此仍需检查外层赋值。
+                            lvalue_type = var.type
+                            self.check_subtype(
+                                rvalue_type,
+                                lvalue_type,
+                                rvalue,
+                                message_registry.INCOMPATIBLE_TYPES_IN_ASSIGNMENT,
+                                "expression has type",
+                                "variable has type",
+                            )
                     else:
                         # Try to infer a partial type.
                         if not self.infer_partial_type(var, lvalue, rvalue_type):

test-data/unit/check-python38.test

@@ -821,6 +821,13 @@ i: Optional[int] = 0 if condition() else None
 x: Optional[tuple[int, int]] = (i, (i := 1))  # E: Incompatible types in assignment (expression has type "tuple[int | None, int]", variable has type "tuple[int, int] | None")
 [builtins fixtures/tuple.pyi]
 
+[case testPartialTypeWalrusAssignment]
+def fn() -> int:
+    x = None
+    x = (x, (x := 1))  # E: Incompatible types in assignment (expression has type "tuple[None, int]", variable has type "int | None")
+    return x
+[builtins fixtures/tuple.pyi]
+
 [case testReturnTupleOptionalWalrus]
 from typing import Optional