Fix narrowing with chained comparison

Fixes #21149

Author: hauntsaninja

mypy/checker.py

@@ -6674,8 +6674,8 @@ def comparison_type_narrowing_helper(self, node: ComparisonExpr) -> tuple[TypeMa
         # If we have found non-trivial restrictions from the regular comparisons,
         # then return soon. Otherwise try to infer restrictions involving `len(x)`.
         # TODO: support regular and len() narrowing in the same chain.
-        if any(m != ({}, {}) for m in partial_type_maps):
-            return reduce_conditional_maps(partial_type_maps)
+        if any(len(m[0]) or len(m[1]) for m in partial_type_maps):
+            return reduce_conditional_maps(partial_type_maps, use_meet=True)
         else:
             # Use meet for `and` maps to get correct results for chained checks
             # like `if 1 < len(x) < 4: ...`

test-data/unit/check-narrowing.test

@@ -3264,6 +3264,20 @@ def bad_but_should_pass(has_key: bool, key: bool, s: tuple[bool, ...]) -> None:
         reveal_type(key)  # N: Revealed type is "builtins.bool"
 [builtins fixtures/primitives.pyi]
 
+[case testNarrowChainedComparisonMeet]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+
+def f1(a: str | None, b: str | None) -> None:
+    if None is not a == b:
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+        reveal_type(b)  # N: Revealed type is "builtins.str | None"
+
+    if (None is not a) and (a == b):
+        reveal_type(a)  # N: Revealed type is "builtins.str"
+        reveal_type(b)  # N: Revealed type is "builtins.str"
+[builtins fixtures/primitives.pyi]
+
 [case testNarrowTypeObject]
 # flags: --strict-equality --warn-unreachable
 from typing import Any