typing.assert_never(5) is fine in unreachable code

This is an awful hack but the documentation says:
> Ask a static type checker to confirm that a line of code is
> unreachable.

as well as something specifically about `Never` (it's confusing)

Author: A5rocks

mypy/checkexpr.py

@@ -1669,6 +1669,12 @@ def check_callable_call(
         if isinstance(callable_node, RefExpr) and callable_node.fullname in ENUM_BASES:
             # An Enum() call that failed SemanticAnalyzerPass2.check_enum_call().
             return callee.ret_type, callee
+        if (
+            isinstance(callable_node, RefExpr)
+            and callable_node.fullname in ("typing.assert_never", "typing_extensions.assert_never")
+            and self.chk.binder.is_unreachable()
+        ):
+            return callee.ret_type, callee
 
         if (
             callee.is_type_obj()

test-data/unit/check-unreachable-code.test

@@ -875,6 +875,8 @@ d = [x for x in lst if FOOBAR]
 [case testUnreachableFlagOkWithDeadStatements]
 # flags: --warn-unreachable
 from typing import NoReturn
+from typing_extensions import assert_never as typing_assert_never
+
 def assert_never(x: NoReturn) -> NoReturn:
     assert False
 
@@ -895,6 +897,7 @@ if False:
 
 if False:
     assert_never(x)  # E: Argument 1 to "assert_never" has incompatible type "int"; expected "Never"
+    typing_assert_never(x)
     reveal_type(x)  # E: Statement is unreachable \
                     # N: Revealed type is "builtins.int"
 
@@ -907,6 +910,14 @@ if False:
     # Ignore obvious type errors
     assert_never(expect_str(x))  # E: Argument 1 to "assert_never" has incompatible type "str"; expected "Never" \
                                  # E: Argument 1 to "expect_str" has incompatible type "int"; expected "str"
+    typing_assert_never(expect_str(x))
+    reveal_type(x)  # E: Statement is unreachable \
+                    # N: Revealed type is "builtins.int"
+
+if True:
+    typing_assert_never(x)  # E: Argument 1 to "assert_never" has incompatible type "int"; expected "Never"
+    assert_never(x)  # E: Argument 1 to "assert_never" has incompatible type "int"; expected "Never"
+    typing_assert_never(x)
     reveal_type(x)  # E: Statement is unreachable \
                     # N: Revealed type is "builtins.int"
 [builtins fixtures/exception.pyi]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -81,6 +81,7 @@ def TypedDict(typename: str, fields: Dict[str, Type[_T]], *, total: Any = ...) -
 
 def reveal_type(__obj: _T) -> _T: pass
 def assert_type(__val: _T, __typ: Any) -> _T: pass
+def assert_never(__val: Never) -> Never: pass
 
 def dataclass_transform(
     *,