Proper subtype checks use type promotions

Author: JukkaL

mypy/subtypes.py

@@ -563,6 +563,10 @@ def visit_deleted_type(self, left: DeletedType) -> bool:
 
     def visit_instance(self, left: Instance) -> bool:
         if isinstance(self.right, Instance):
+            for base in left.type.mro:
+                if base._promote and is_proper_subtype(base._promote, self.right):
+                    return True
+
             if not left.type.has_base(self.right.type.fullname()):
                 return False
 
@@ -579,6 +583,7 @@ def check_argument(leftarg: Type, rightarg: Type, variance: int) -> bool:
 
             return all(check_argument(ta, ra, tvar.variance) for ta, ra, tvar in
                        zip(left.args, self.right.args, self.right.type.defn.type_vars))
+        # TODO: TypeType
         return False
 
     def visit_type_var(self, left: TypeVarType) -> bool:

test-data/unit/check-unions.test

@@ -295,3 +295,15 @@ def foo(a: Union[A, B, C]):
         a.y # E: Some element of union has no attribute "y"
         b = a # type: Union[B, C]
 [builtins fixtures/isinstance.pyi]
+
+[case testSimplifyingUnionAndTypePromotions]
+from typing import TypeVar, Union
+T = TypeVar('T')
+S = TypeVar('S')
+def u(x: T, y: S) -> Union[S, T]: pass
+
+reveal_type(u(1, 2.3))  # E: Revealed type is 'builtins.float*'
+reveal_type(u(2.3, 1))  # E: Revealed type is 'builtins.float*'
+reveal_type(u(False, 2.2)) # E: Revealed type is 'builtins.float*'
+reveal_type(u(2.2, False)) # E: Revealed type is 'builtins.float*'
+[builtins fixtures/primitives.pyi]

test-data/unit/fixtures/primitives.pyi

@@ -11,7 +11,7 @@ class int:
     def __add__(self, i: int) -> int: pass
 class float: pass
 class complex: pass
-class bool: pass
+class bool(int): pass
 class str:
     def __add__(self, s: str) -> str: pass
     def format(self, *args) -> str: pass