Add --strict-bool flag to prohib treating bool as int

docs/source/command_line.rst

@@ -642,6 +642,26 @@ of the above sections.
 
        assert text is not None  # OK, check against None is allowed as a special case.
 
+.. option:: --strict-bool
+
+    By default ``bool`` values are treated as subtypes of ``int``,
+    just like in runtime:
+
+    .. code-block:: python
+
+        >>> bool.__mro__
+        (<class 'bool'>, <class 'int'>, <class 'object'>)
+
+    While it will work in runtime,
+    some cases might require a little bit more strictness.
+    With this flag enabled, you will get the following error:
+
+    .. code-block:: python
+
+        def requires_int(arg: int) -> None: ...
+
+        requires_int(5 > 0)  # Error: Argument 1 has incompatible type "bool"; expected "int"
+
 .. option:: --extra-checks
 
     This flag enables additional checks that are technically correct but may be

mypy/main.py

@@ -843,6 +843,14 @@ def add_invertible_flag(
         group=strictness_group,
     )
 
+    add_invertible_flag(
+        "--strict-bool",
+        default=False,
+        strict_flag=True,
+        help="Prohib to treat bool as int",
+        group=strictness_group,
+    )
+
     add_invertible_flag(
         "--extra-checks",
         default=False,

mypy/options.py

@@ -50,6 +50,7 @@ class BuildType:
     "mypyc",
     "strict_concatenate",
     "strict_equality",
+    "strict_bool",
     "strict_optional",
     "warn_no_return",
     "warn_return_any",
@@ -209,6 +210,10 @@ def __init__(self) -> None:
         # This makes 1 == '1', 1 in ['1'], and 1 is '1' errors.
         self.strict_equality = False
 
+        # Prohibit to treat `bool` as `int` in subtyping contexts.
+        # This makes `def a(b: int): ...; a(True)` an error.
+        self.strict_bool = False
+
         # Deprecated, use extra_checks instead.
         self.strict_concatenate = False
 

mypy/subtypes.py

@@ -512,6 +512,13 @@ def visit_instance(self, left: Instance) -> bool:
                 if left.type.alt_promote and left.type.alt_promote.type is right.type:
                     return True
             rname = right.type.fullname
+            if (
+                self.options
+                and self.options.strict_bool
+                and left.type.fullname == "builtins.bool"
+                and rname == "builtins.int"
+            ):
+                return False
             # Always try a nominal check if possible,
             # there might be errors that a user wants to silence *once*.
             # NamedTuples are a special case, because `NamedTuple` is not listed

test-data/unit/check-flags.test

@@ -2284,3 +2284,37 @@ class C(Generic[T]): ...
 
 A = Union[C, List]  # OK
 [builtins fixtures/list.pyi]
+
+[case testStrictBool]
+# flags: --strict-bool --show-error-codes
+from typing import List, Union
+
+def a(x: int): ...
+a(True)  # E: Argument 1 to "a" has incompatible type "bool"; expected "int"  [arg-type]
+a(False) # E: Argument 1 to "a" has incompatible type "bool"; expected "int"  [arg-type]
+
+bl: bool
+a(bl)    # E: Argument 1 to "a" has incompatible type "bool"; expected "int"  [arg-type]
+
+def b() -> int:
+    return bl  # E: Incompatible return value type (got "bool", expected "int")  [return-value]
+
+c: List[int] = [
+    True,  # E: List item 0 has incompatible type "bool"; expected "int"  [list-item]
+    False, # E: List item 1 has incompatible type "bool"; expected "int"  [list-item]
+    bl,    # E: List item 2 has incompatible type "bool"; expected "int"  [list-item]
+]
+
+# OK:
+def d(x: Union[int, bool], y: bool): ...
+d(1, True)
+d(True, False)
+d(bl, bl)
+[builtins fixtures/list.pyi]
+
+[case testStrictBoolWithStrictFlag]
+# flags: --strict
+
+def a(x: int) -> None: ...
+b: bool
+a(b)  # E: Argument 1 to "a" has incompatible type "bool"; expected "int"