Don't consider a branch unreachable if there is possible promotion

mypy/checker.py

@@ -4850,6 +4850,17 @@ def intersect_instances(
         curr_module = self.scope.stack[0]
         assert isinstance(curr_module, MypyFile)
 
+        # First, retry narrowing while allowing promotions (they are disabled by default
+        # for isinstance() checks, etc). This way we will still type-check branches like
+        # x: complex = 1
+        # if isinstance(x, int):
+        #     ...
+        left, right = instances
+        if is_proper_subtype(left, right, ignore_promotions=False):
+            return left
+        if is_proper_subtype(right, left, ignore_promotions=False):
+            return right
+
         def _get_base_classes(instances_: tuple[Instance, Instance]) -> list[Instance]:
             base_classes_ = []
             for inst in instances_:

test-data/unit/check-type-promotion.test

@@ -54,3 +54,59 @@ def f(x: Union[SupportsFloat, T]) -> Union[SupportsFloat, T]: pass
 f(0)  # should not crash
 [builtins fixtures/primitives.pyi]
 [out]
+
+[case testIntersectionUsingPromotion]
+# flags: --warn-unreachable
+x: complex = 1
+reveal_type(x)  # N: Revealed type is "builtins.complex"
+if isinstance(x, int):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+else:
+    reveal_type(x)  # N: Revealed type is "builtins.complex"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion2]
+# flags: --warn-unreachable
+x: complex = 1
+reveal_type(x)  # N: Revealed type is "builtins.complex"
+if isinstance(x, (int, float)):
+    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float]"
+else:
+    reveal_type(x)  # N: Revealed type is "builtins.complex"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion3]
+# flags: --warn-unreachable
+x: object
+if isinstance(x, int) and isinstance(x, complex):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+if isinstance(x, complex) and isinstance(x, int):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion4]
+# flags: --warn-unreachable
+x: object
+if isinstance(x, int):
+    if isinstance(x, complex):
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+if isinstance(x, complex):
+    if isinstance(x, int):
+        reveal_type(x)  # N: Revealed type is "builtins.int"
+    else:
+        reveal_type(x)  # N: Revealed type is "builtins.complex"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion5]
+# flags: --warn-unreachable
+from typing import Union
+
+x: Union[float, complex]
+if isinstance(x, int):
+    # Most likely this was an error, but we still type-check this branch
+    reveal_type(x)  # N: Revealed type is "<nothing>"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
+[builtins fixtures/primitives.pyi]

test-data/unit/fixtures/primitives.pyi

@@ -1,5 +1,5 @@
 # builtins stub with non-generic primitive types
-from typing import Generic, TypeVar, Sequence, Iterator, Mapping, Iterable, overload
+from typing import Generic, TypeVar, Sequence, Iterator, Mapping, Iterable, Tuple, Union
 
 T = TypeVar('T')
 V = TypeVar('V')
@@ -20,7 +20,9 @@ class int:
     def __rmul__(self, x: int) -> int: pass
 class float:
     def __float__(self) -> float: pass
-class complex: pass
+    def __add__(self, x: float) -> float: ...
+class complex:
+    def __add__(self, x: complex) -> complex: ...
 class bool(int): pass
 class str(Sequence[str]):
     def __add__(self, s: str) -> str: pass
@@ -63,3 +65,5 @@ class range(Sequence[int]):
     def __getitem__(self, i: int) -> int: pass
     def __iter__(self) -> Iterator[int]: pass
     def __contains__(self, other: object) -> bool: pass
+
+def isinstance(x: object, t: Union[type, Tuple]) -> bool: pass