Fix false positive redundant-expr for 'if x and y' when body always raises

mypy/checker.py

@@ -5345,6 +5345,11 @@ def visit_if_stmt(self, s: IfStmt) -> None:
         # Fall-through to the original frame is handled explicitly in each block.
         with self.binder.frame_context(can_skip=False, conditional_frame=True, fall_through=0):
             for e, b in zip(s.expr, s.body):
+                # If condition is 'x and y' and body always raises,
+                # suppress false redundant-expr warning (see issue #21533)
+                if isinstance(e, OpExpr) and e.op == "and":
+                    if b.body and all(self.is_noop_for_reachability(s) for s in b.body):
+                        e.right_always = True
                 t = get_proper_type(self.expr_checker.accept(e))
 
                 if isinstance(t, DeletedType):

test-data/unit/check-narrowing.test

@@ -4281,3 +4281,27 @@ def func(y: H) -> H:
     else:
         return y
 [builtins fixtures/primitives.pyi]
+
+[case testRedundantExprAndWithRaise]
+# flags: --enable-error-code redundant-expr
+from typing_extensions import TypeIs
+from typing import TypeVar, Sequence, Any
+
+class Cat: ...
+
+T = TypeVar('T')
+MyType = TypeVar('MyType', int, str, Cat)
+
+def _is_seq_of(seq: Sequence[Any], tp: type[T]) -> TypeIs[Sequence[T]]: ...
+
+def main1(a: Sequence[MyType]) -> MyType:
+    if not _is_seq_of(a, Cat) and not _is_seq_of(a, int):
+        raise Exception("unexpected")
+    return a[0]
+
+def main2(a: Sequence[MyType]) -> MyType:
+    if not _is_seq_of(a, Cat):
+        if not _is_seq_of(a, int):
+            raise Exception("unexpected")
+    return a[0]
+[builtins fixtures/exception.pyi]