Narrow walrus to Never if unreachable

Author: A5rocks

mypy/checker.py

@@ -6228,12 +6228,17 @@ def find_isinstance_check_helper(
 
             if if_condition_map is not None:
                 if_map.update(if_condition_map)
+            else:
+                if_map[node.target] = UninhabitedType()
+
             if else_condition_map is not None:
                 else_map.update(else_condition_map)
+            else:
+                else_map[node.target] = UninhabitedType()
 
             return (
-                (None if if_assignment_map is None or if_condition_map is None else if_map),
-                (None if else_assignment_map is None or else_condition_map is None else else_map),
+                (None if if_assignment_map is None else if_map),
+                (None if else_assignment_map is None else else_map),
             )
         elif isinstance(node, OpExpr) and node.op == "and":
             left_if_vars, left_else_vars = self.find_isinstance_check(node.left)

test-data/unit/check-python38.test

@@ -460,7 +460,7 @@ def check_partial_list() -> None:
 
 if (x := 0):
     reveal_type(x)  # E: Statement is unreachable \
-                    # N: Revealed type is "builtins.int"
+                    # N: Revealed type is "Never"
 else:
     reveal_type(x)  # N: Revealed type is "Literal[0]"