Make NoReturn a SpecialForm (#6290)

This is consistent with the runtime definition: https://github.com/python/cpython/blob/main/Lib/typing.py#L434.

The previous definition was wrong; NoReturn is not and should not be equivalent to None. This fixes an issue in pyanalyze where it was interpreting NoReturn as equivalent to None.

Author: JelleZijlstra

.github/workflows/tests.yml

@@ -89,7 +89,7 @@ jobs:
         python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
       fail-fast: false
     env:
-      PYRIGHT_VERSION: 1.1.184 # Must match pyright_test.py.
+      PYRIGHT_VERSION: 1.1.187 # Must match pyright_test.py.
     steps:
       - uses: actions/checkout@v2
       - uses: jakebailey/pyright-action@v1

stdlib/typing.pyi

@@ -50,6 +50,7 @@ Protocol: _SpecialForm = ...
 Callable: _SpecialForm = ...
 Type: _SpecialForm = ...
 ClassVar: _SpecialForm = ...
+NoReturn: _SpecialForm = ...
 if sys.version_info >= (3, 8):
     Final: _SpecialForm = ...
     def final(f: _T) -> _T: ...
@@ -83,11 +84,6 @@ if sys.version_info >= (3, 10):
     TypeAlias: _SpecialForm = ...
     TypeGuard: _SpecialForm = ...
 
-# Return type that indicates a function does not return.
-# This type is equivalent to the None type, but the no-op Union is necessary to
-# distinguish the None type from the None value.
-NoReturn = Union[None]
-
 # These type variables are used by the container types.
 _S = TypeVar("_S")
 _KT = TypeVar("_KT")  # Key type.

tests/pyright_test.py

@@ -5,7 +5,7 @@
 import sys
 from pathlib import Path
 
-_PYRIGHT_VERSION = "1.1.184"  # Must match tests.yml.
+_PYRIGHT_VERSION = "1.1.187"  # Must match tests.yml.
 _WELL_KNOWN_FILE = Path("tests", "pyright_test.py")