Add NoReturn (#811)

* Add NoReturn

stdlib/3/builtins.pyi

@@ -10,6 +10,7 @@ from typing import (
 from abc import abstractmethod, ABCMeta
 from types import TracebackType
 import sys
+from mypy_extensions import NoReturn
 
 # Note that names imported above are not automatically made visible via the
 # implicit builtins import.
@@ -697,7 +698,7 @@ def eval(source: str, globals: Dict[str, Any] = None,
          locals: Mapping[str, Any] = None) -> Any: ...  # TODO code object as source
 def exec(object: str, globals: Dict[str, Any] = None,
          locals: Mapping[str, Any] = None) -> Any: ...  # TODO code object as source
-def exit(code: int = None) -> None: ...
+def exit(code: int = None) -> NoReturn: ...
 @overload
 def filter(function: Callable[[_T], Any], iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload

stdlib/3/os/__init__.pyi

@@ -11,6 +11,7 @@ from typing import (
     Optional, Generic, Set, Callable
 )
 from . import path
+from mypy_extensions import NoReturn
 
 # ----- os variables -----
 
@@ -314,7 +315,7 @@ def execve(path: AnyStr, args: Union[Tuple[AnyStr], List[AnyStr]], env: Mapping[
 def execvp(file: AnyStr, args: Union[Tuple[AnyStr], List[AnyStr]]) -> None: ...
 def execvpe(file: AnyStr, args: Union[Tuple[AnyStr], List[AnyStr]],
             env: Mapping[str, str]) -> None: ...
-def _exit(n: int) -> None: ...
+def _exit(n: int) -> NoReturn: ...
 def fork() -> int: ...  # Unix only
 def forkpty() -> Tuple[int, int]: ...  # some flavors of Unix
 def kill(pid: int, sig: int) -> None: ...

stdlib/3/sys.pyi

@@ -8,6 +8,7 @@ from typing import (
     TypeVar, Callable, Type,
 )
 from types import TracebackType
+from mypy_extensions import NoReturn
 
 _T = TypeVar('_T')
 
@@ -120,7 +121,7 @@ def exc_info() -> Tuple[Optional[Type[BaseException]],
                         Optional[BaseException],
                         Optional[TracebackType]]: ...
 # sys.exit() accepts an optional argument of anything printable
-def exit(arg: Any = ...) -> None:
+def exit(arg: Any = ...) -> NoReturn:
     raise SystemExit()
 def getcheckinterval() -> int: ...  # deprecated
 def getdefaultencoding() -> str: ...