Fix type narrowing for variable swaps after isinstance checks

mypy/checker.py

@@ -4175,7 +4175,19 @@ def check_assignment_to_multiple_lvalues(
                     lr_pairs.append((star_lv.expr, rv_list))
                 lr_pairs.extend(zip(right_lvs, right_rvs))
 
+                captured_pairs: list[tuple[Lvalue, Expression]] = []
+
                 for lv, rv in lr_pairs:
+                    if isinstance(rv, NameExpr):
+                        narrowed = self.binder.get(rv)
+
+                        if narrowed is not None:
+                            captured_pairs.append((lv, self.temp_node(narrowed, context=rv)))
+                            continue
+
+                    captured_pairs.append((lv, rv))
+
+                for lv, rv in captured_pairs:
                     self.check_assignment(lv, rv, infer_lvalue_type)
         else:
             self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)

test-data/unit/check-isinstance.test

@@ -3377,3 +3377,21 @@ def f2(x: A | None, t: type[A]):
     else:
         reveal_type(x)  # N: Revealed type is "None"
 [builtins fixtures/isinstancelist.pyi]
+
+[case testTupleSwapAfterIsinstance]
+class Base: pass
+class Sub1(Base): pass
+class Sub2(Base): pass
+
+def f(a: Base, b: Base) -> None:
+    if isinstance(b, Sub1) and isinstance(a, Sub2):
+        t = (b, a)
+
+        reveal_type(t)  # N: Revealed type is "tuple[__main__.Sub1, __main__.Sub2]"
+
+        a, b = t
+
+        reveal_type(a)  # N: Revealed type is "__main__.Sub1"
+        reveal_type(b)  # N: Revealed type is "__main__.Sub2"
+
+[builtins fixtures/isinstancelist.pyi]

test-data/unit/check-narrowing.test

@@ -4281,3 +4281,21 @@ def func(y: H) -> H:
     else:
         return y
 [builtins fixtures/primitives.pyi]
+
+[case testIsInstanceVariableSwapNarrowing]
+class Base:
+    pass
+
+class Sub1(Base):
+    pass
+
+class Sub2(Base):
+    pass
+
+def f(a: Base, b: Base) -> None:
+    if isinstance(b, Sub1) and isinstance(a, Sub2):
+        a, b = b, a
+        reveal_type(a)  # N: Revealed type is "__main__.Sub1"
+        reveal_type(b)  # N: Revealed type is "__main__.Sub2"
+
+[builtins fixtures/isinstancelist.pyi]