Second expression in assert statement should be checked as conditional

mypy/checker.py

@@ -2979,14 +2979,13 @@ def visit_operator_assignment_stmt(self,
     def visit_assert_stmt(self, s: AssertStmt) -> None:
         self.expr_checker.accept(s.expr)
 
-        if s.msg is not None:
-            self.expr_checker.accept(s.msg)
-
         if isinstance(s.expr, TupleExpr) and len(s.expr.items) > 0:
             self.fail(message_registry.MALFORMED_ASSERT, s)
 
         # If this is asserting some isinstance check, bind that type in the following code
-        true_map, _ = self.find_isinstance_check(s.expr)
+        true_map, else_map = self.find_isinstance_check(s.expr)
+        if s.msg is not None:
+            self.expr_checker.analyze_cond_branch(else_map, s.msg, None)
         self.push_type_map(true_map)
 
     def visit_raise_stmt(self, s: RaiseStmt) -> None:

test-data/unit/check-expressions.test

@@ -2331,3 +2331,22 @@ def f(x: any) -> None:  # E: Name 'any' is not defined \
 def f(x: Optional[str]) -> None:  # E: Name 'Optional' is not defined \
                                   # N: Did you forget to import it from "typing"? (Suggestion: "from typing import Optional")
     pass
+
+[case testAssertionLazilyWithIsNone]
+from typing import Optional, List
+li: Optional[List] = []
+assert li is None, li[0]
+[builtins fixtures/list.pyi]
+
+
+[case testAssertionLazilyWithIsInstance]
+from typing import Optional, List
+li: Optional[List] = []
+assert not isinstance(li,list), li[0]
+[builtins fixtures/isinstancelist.pyi]
+
+[case testAssertCurrentFrameIsNotUnreachable]
+def f() -> int:  # E: Missing return statement
+    x: int
+    assert isinstance(x, int), '...'
+[builtins fixtures/isinstance.pyi]