Check for truthy-bool in not ... unary expressions (#17773)

Closes #17769

mypy/checker.py

@@ -5651,7 +5651,16 @@ def _is_truthy_type(self, t: ProperType) -> bool:
             )
         )
 
-    def _check_for_truthy_type(self, t: Type, expr: Expression) -> None:
+    def check_for_truthy_type(self, t: Type, expr: Expression) -> None:
+        """
+        Check if a type can have a truthy value.
+
+        Used in checks like::
+
+            if x: # <---
+
+            not x  # <---
+        """
         if not state.strict_optional:
             return  # if everything can be None, all bets are off
 
@@ -6145,7 +6154,7 @@ def has_no_custom_eq_checks(t: Type) -> bool:
         if in_boolean_context:
             # We don't check `:=` values in expressions like `(a := A())`,
             # because they produce two error messages.
-            self._check_for_truthy_type(original_vartype, node)
+            self.check_for_truthy_type(original_vartype, node)
         vartype = try_expanding_sum_type_to_union(original_vartype, "builtins.bool")
 
         if_type = true_only(vartype)

mypy/checkexpr.py

@@ -4256,6 +4256,7 @@ def visit_unary_expr(self, e: UnaryExpr) -> Type:
         op = e.op
         if op == "not":
             result: Type = self.bool_type()
+            self.chk.check_for_truthy_type(operand_type, e.expr)
         else:
             method = operators.unary_op_methods[op]
             result, method_type = self.check_method_call_by_name(method, operand_type, [], [], e)

test-data/unit/check-errorcodes.test

@@ -846,53 +846,75 @@ 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]
     pass
 
+not 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]
+
 zero = 0
 if zero:
     pass
 
+not zero
+
 false = False
 if false:
     pass
 
+not false
+
 null = None
 if null:
     pass
 
+not null
+
 s = ''
 if s:
     pass
 
+not s
+
 good_union: Union[str, int] = 5
 if good_union:
     pass
 if not good_union:
     pass
 
+not good_union
+
 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
 
+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]
+
 # 'object' is special and is treated as potentially falsy
 obj: object = Foo()
 if obj:
     pass
 if not obj:
     pass
 
+not obj
+
 lst: List[int] = []
 if lst:
     pass
 
+not lst
+
 a: Any
 if a:
     pass
 
+not a
+
 any_or_object: Union[object, Any]
 if any_or_object:
     pass
 
+not any_or_object
+
 if (my_foo := Foo()):  # E: "__main__.my_foo" has type "Foo" which does not implement __bool__ or __len__ so it could always be true in boolean context  [truthy-bool]
     pass
 
@@ -909,13 +931,17 @@ if not f:  # E: Function "f" could always be true in boolean context  [truthy-fu
     pass
 conditional_result = 'foo' if f else 'bar'  # E: Function "f" could always be true in boolean context  [truthy-function]
 
+not f  # E: Function "f" could always be true in boolean context  [truthy-function]
+
 [case testTruthyIterable]
 # flags: --enable-error-code truthy-iterable
 from typing import Iterable
 def func(var: Iterable[str]) -> None:
     if var:  # E: "var" has type "Iterable[str]" which can always be true in boolean context. Consider using "Collection[str]" instead.  [truthy-iterable]
         ...
 
+    not var  # E: "var" has type "Iterable[str]" which can always be true in boolean context. Consider using "Collection[str]" instead.  [truthy-iterable]
+
 [case testNoOverloadImplementation]
 from typing import overload