Handle NoReturn type aliases (#11912)

Author: sobolevn

mypy/semanal.py

@@ -2245,6 +2245,12 @@ def is_type_ref(self, rv: Expression, bare: bool = False) -> bool:
                 return True
             # Assignment color = Color['RED'] defines a variable, not an alias.
             return not rv.node.is_enum
+        if isinstance(rv.node, Var):
+            return rv.node.fullname in (
+                'typing.NoReturn',
+                'typing_extensions.NoReturn',
+                'mypy_extensions.NoReturn',
+            )
 
         if isinstance(rv, NameExpr):
             n = self.lookup(rv.name, rv)

test-data/unit/check-type-aliases.test

@@ -50,6 +50,14 @@ def f(x: A) -> None:
 f(1)
 f('x')
 
+[case testNoReturnTypeAlias]
+# https://github.com/python/mypy/issues/11903
+from typing import NoReturn
+Never = NoReturn
+a: Never  # Used to be an error here
+
+def f(a: Never): ...
+f(5)  # E: Argument 1 to "f" has incompatible type "int"; expected "NoReturn" 
 [case testImportUnionAlias]
 import typing
 from _m import U