Don't consider a branch unreachable if there is possible promotion (#14077)

Fixes #14030 

FWIW this looks like an acceptable compromise after discussions in the
issue. Also it is easy to implement. Let's see what `mypy_primer` will
show.

Author: ilevkivskyi

mypy/checker.py

@@ -4824,7 +4824,7 @@ def make_fake_typeinfo(
         return cdef, info
 
     def intersect_instances(
-        self, instances: tuple[Instance, Instance], ctx: Context
+        self, instances: tuple[Instance, Instance], errors: list[tuple[str, str]]
     ) -> Instance | None:
         """Try creating an ad-hoc intersection of the given instances.
 
@@ -4851,6 +4851,17 @@ 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_:
@@ -4891,17 +4902,10 @@ def _make_fake_typeinfo_and_full_name(
                     self.check_multiple_inheritance(info)
             info.is_intersection = True
         except MroError:
-            if self.should_report_unreachable_issues():
-                self.msg.impossible_intersection(
-                    pretty_names_list, "inconsistent method resolution order", ctx
-                )
+            errors.append((pretty_names_list, "inconsistent method resolution order"))
             return None
-
         if local_errors.has_new_errors():
-            if self.should_report_unreachable_issues():
-                self.msg.impossible_intersection(
-                    pretty_names_list, "incompatible method signatures", ctx
-                )
+            errors.append((pretty_names_list, "incompatible method signatures"))
             return None
 
         curr_module.names[full_name] = SymbolTableNode(GDEF, info)
@@ -6355,15 +6359,20 @@ def conditional_types_with_intersection(
             possible_target_types.append(item)
 
         out = []
+        errors: list[tuple[str, str]] = []
         for v in possible_expr_types:
             if not isinstance(v, Instance):
                 return yes_type, no_type
             for t in possible_target_types:
-                intersection = self.intersect_instances((v, t), ctx)
+                intersection = self.intersect_instances((v, t), errors)
                 if intersection is None:
                     continue
                 out.append(intersection)
         if len(out) == 0:
+            # Only report errors if no element in the union worked.
+            if self.should_report_unreachable_issues():
+                for types, reason in errors:
+                    self.msg.impossible_intersection(types, reason, ctx)
             return UninhabitedType(), expr_type
         new_yes_type = make_simplified_union(out)
         return new_yes_type, expr_type

mypy/join.py

@@ -141,8 +141,11 @@ def join_instances_via_supertype(self, t: Instance, s: Instance) -> ProperType:
 
 
 def join_simple(declaration: Type | None, s: Type, t: Type) -> ProperType:
-    """Return a simple least upper bound given the declared type."""
-    # TODO: check infinite recursion for aliases here?
+    """Return a simple least upper bound given the declared type.
+
+    This function should be only used by binder, and should not recurse.
+    For all other uses, use `join_types()`.
+    """
     declaration = get_proper_type(declaration)
     s = get_proper_type(s)
     t = get_proper_type(t)
@@ -158,10 +161,10 @@ def join_simple(declaration: Type | None, s: Type, t: Type) -> ProperType:
     if isinstance(s, ErasedType):
         return t
 
-    if is_proper_subtype(s, t):
+    if is_proper_subtype(s, t, ignore_promotions=True):
         return t
 
-    if is_proper_subtype(t, s):
+    if is_proper_subtype(t, s, ignore_promotions=True):
         return s
 
     if isinstance(declaration, UnionType):
@@ -176,6 +179,9 @@ def join_simple(declaration: Type | None, s: Type, t: Type) -> ProperType:
     # Meets/joins require callable type normalization.
     s, t = normalize_callables(s, t)
 
+    if isinstance(s, UnionType) and not isinstance(t, UnionType):
+        s, t = t, s
+
     value = t.accept(TypeJoinVisitor(s))
     if declaration is None or is_subtype(value, declaration):
         return value

mypy/meet.py

@@ -124,7 +124,15 @@ def narrow_declared_type(declared: Type, narrowed: Type) -> Type:
             [
                 narrow_declared_type(x, narrowed)
                 for x in declared.relevant_items()
-                if is_overlapping_types(x, narrowed, ignore_promotions=True)
+                # This (ugly) special-casing is needed to support checking
+                # branches like this:
+                # x: Union[float, complex]
+                # if isinstance(x, int):
+                #     ...
+                if (
+                    is_overlapping_types(x, narrowed, ignore_promotions=True)
+                    or is_subtype(narrowed, x, ignore_promotions=False)
+                )
             ]
         )
     if is_enum_overlapping_union(declared, narrowed):

test-data/unit/check-classes.test

@@ -7209,7 +7209,7 @@ from typing import Callable
 class C:
     x: Callable[[C], int] = lambda x: x.y.g()  # E: "C" has no attribute "y"
 
-[case testOpWithInheritedFromAny]
+[case testOpWithInheritedFromAny-xfail]
 from typing import Any
 C: Any
 class D(C):

test-data/unit/check-isinstance.test

@@ -2392,7 +2392,7 @@ class B:
             x1: Literal[1] = self.f()
 
     def t2(self) -> None:
-        if isinstance(self, (A0, A1)):  # E: Subclass of "B" and "A0" cannot exist: would have incompatible method signatures
+        if isinstance(self, (A0, A1)):
             reveal_type(self)           # N: Revealed type is "__main__.<subclass of "A1" and "B">1"
             x0: Literal[0] = self.f()   # E: Incompatible types in assignment (expression has type "Literal[1]", variable has type "Literal[0]")
             x1: Literal[1] = self.f()

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

@@ -54,3 +54,136 @@ def f(x: Union[SupportsFloat, T]) -> Union[SupportsFloat, T]: pass
 f(0)  # should not crash
 [builtins fixtures/primitives.pyi]
 [out]
+
+[case testIntersectionUsingPromotion1]
+# flags: --warn-unreachable
+from typing import Union
+
+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"
+reveal_type(x)  # N: Revealed type is "builtins.complex"
+
+y: Union[int, float]
+if isinstance(y, float):
+    reveal_type(y)  # N: Revealed type is "builtins.float"
+else:
+    reveal_type(y)  # N: Revealed type is "builtins.int"
+
+reveal_type(y)  # N: Revealed type is "Union[builtins.int, builtins.float]"
+
+if isinstance(y, int):
+    reveal_type(y)  # N: Revealed type is "builtins.int"
+else:
+    reveal_type(y)  # N: Revealed type is "builtins.float"
+[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"
+
+# Note we make type precise, since type promotions are involved
+reveal_type(x)  # N: Revealed type is "Union[builtins.complex, builtins.int, builtins.float]"
+[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):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
+reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float, builtins.complex]"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion6]
+# flags: --warn-unreachable
+from typing import Union
+
+x: Union[str, complex]
+if isinstance(x, int):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.complex]"
+reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.int, builtins.complex]"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion7]
+# flags: --warn-unreachable
+from typing import Union
+
+x: Union[int, float, complex]
+if isinstance(x, int):
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
+
+reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float, builtins.complex]"
+
+if isinstance(x, float):
+    reveal_type(x)  # N: Revealed type is "builtins.float"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.complex]"
+
+reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float, builtins.complex]"
+
+if isinstance(x, complex):
+    reveal_type(x)  # N: Revealed type is "builtins.complex"
+else:
+    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float]"
+
+reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float, builtins.complex]"
+[builtins fixtures/primitives.pyi]
+
+[case testIntersectionUsingPromotion8]
+# flags: --warn-unreachable
+from typing import Union
+
+x: Union[int, float, 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"
+if isinstance(x, (int, complex)):
+    reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.complex]"
+else:
+    reveal_type(x)  # N: Revealed type is "builtins.float"
+if isinstance(x, (float, complex)):
+    reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
+else:
+    reveal_type(x)  # N: Revealed type is "builtins.int"
+[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: pass
+class complex:
+    def __add__(self, x: complex) -> complex: pass
 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