Don't consider 'object' always truthy (#14510)

There are two reasons I'm proposing this change. First, we know that
many subclasses of 'object' can be falsy. Second, mypy sometimes
simplifies `object | Any` into just `object`. The latter was considered
always truthy, while the prior one wasn't. Now both of them are treated
consistently. An alternative fix would be to not simplify unions like
`object | Any`, but this seems a bit ad hoc.

This only has an effect when the `truthy-bool` error code is explicitly
enabled.

Fixes #14480. This doesn't just fix the regression but fixes a more
general issue.

Author: JukkaL

mypy/checker.py

@@ -5151,6 +5151,7 @@ def _is_truthy_type(self, t: ProperType) -> bool:
                 and bool(t.type)
                 and not t.type.has_readable_member("__bool__")
                 and not t.type.has_readable_member("__len__")
+                and t.type.fullname != "builtins.object"
             )
             or isinstance(t, FunctionLike)
             or (

test-data/unit/check-errorcodes.test

@@ -803,12 +803,15 @@ from typing_extensions import TypedDict
 
 Foo = TypedDict("Bar", {})  # E: First argument "Bar" to TypedDict() does not match variable name "Foo"  [name-match]
 [builtins fixtures/dict.pyi]
+
 [case testTruthyBool]
 # flags: --enable-error-code truthy-bool
-from typing import List, Union
+from typing import List, Union, Any
 
 class Foo:
     pass
+class Bar:
+    pass
 
 foo = Foo()
 if foo:  # E: "__main__.foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
@@ -836,15 +839,30 @@ if good_union:
 if not good_union:
     pass
 
-bad_union: Union[Foo, object] = Foo()
-if bad_union:  # E: "__main__.bad_union" has type "Union[Foo, object]" of which no members implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+bad_union: Union[Foo, Bar] = Foo()
+if bad_union:  # E: "__main__.bad_union" has type "Union[Foo, Bar]" of which no members implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
+if not bad_union:  # E: "__main__.bad_union" has type "Union[Foo, Bar]" of which no members implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+    pass
+
+# 'object' is special and is treated as potentially falsy
+obj: object = Foo()
+if obj:
     pass
-if not bad_union:  # E: "__main__.bad_union" has type "object" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
+if not obj:
     pass
 
 lst: List[int] = []
 if lst:
     pass
+
+a: Any
+if a:
+    pass
+
+any_or_object: Union[object, Any]
+if any_or_object:
+    pass
 [builtins fixtures/list.pyi]
 
 [case testTruthyFunctions]