Make is_subtype and is_proper_subtype do promotion the same way

Fixes #1850.

mypy/subtypes.py

@@ -128,10 +128,11 @@ def visit_instance(self, left: Instance) -> bool:
         if isinstance(right, TupleType) and right.fallback.type.is_enum:
             return is_subtype(left, right.fallback)
         if isinstance(right, Instance):
-            if left.type._promote and is_subtype(
-                    left.type._promote, self.right, self.check_type_parameter,
-                    ignore_pos_arg_names=self.ignore_pos_arg_names):
-                return True
+            for base in left.type.mro:
+                if base._promote and is_subtype(
+                        base._promote, self.right, self.check_type_parameter,
+                        ignore_pos_arg_names=self.ignore_pos_arg_names):
+                    return True
             rname = right.type.fullname()
             if not left.type.has_base(rname) and rname != 'builtins.object':
                 return False

test-data/unit/check-unions.test

@@ -436,3 +436,11 @@ class D:
     def __le__(self, other) -> int: ...
 class E:
     def __ge__(self, other: Union[C, D]) -> int: ...
+
+[case testUnionSimplificationWithBoolIntAndFloat]
+from typing import List, Union
+l = reveal_type([]) # type: List[Union[bool, int, float]] \
+    # E: Revealed type is 'builtins.list[builtins.float]'
+reveal_type(l) \
+    # E: Revealed type is 'builtins.list[Union[builtins.bool, builtins.int, builtins.float]]'
+[builtins fixtures/list.pyi]

test-data/unit/fixtures/list.pyi

@@ -26,7 +26,8 @@ class list(Iterable[T], Generic[T]):
 class tuple: pass
 class function: pass
 class int: pass
+class float: pass
 class str: pass
-class bool: pass
+class bool(int): pass
 
 property = object() # Dummy definition.