From 54d645ee95e9220745cd65160fb720aedf5cb9b6 Mon Sep 17 00:00:00 2001
From: DetachHead <detachhead@users.noreply.github.com>
Date: Sat, 8 Jan 2022 03:33:48 +1000
Subject: [PATCH] add `issubform` support for `Never` and ban instanciating it
 at runtime

---
 basedtyping/__init__.py               | 18 ++++++++++++++----
 tests/test_never_type/test_runtime.py | 17 ++++++++++++++++-
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/basedtyping/__init__.py b/basedtyping/__init__.py
index 2891017..c68e5d5 100644
--- a/basedtyping/__init__.py
+++ b/basedtyping/__init__.py
@@ -11,6 +11,7 @@
     Sequence,
     TypeVar,
     _GenericAlias,
+    _SpecialForm,
     cast,
 )
 
@@ -26,6 +27,7 @@
     """
 
     Never = NoReturn
+    """a type that can never exist. this is the narrowest possible type"""
 else:
     # for isinstance checks
     Function = Callable
@@ -34,6 +36,9 @@ class _NeverMetaclass(type):
         def __instancecheck__(cls, instance: object) -> bool:
             return False
 
+        def __call__(cls) -> Never:
+            raise TypeError("cannot instanciate Never")
+
     class Never(metaclass=_NeverMetaclass):
         pass
 
@@ -223,8 +228,10 @@ def __class_getitem__(cls, item) -> type[ReifiedGeneric[T]]:
 
 # TODO: make this work with any "form", not just unions
 #  should be (form: TypeForm, forminfo: TypeForm)
-def issubform(form: type | UnionType, forminfo: type | UnionType) -> bool:
-    """EXPERIMENTAL: Warning, this function currently only supports unions.
+def issubform(
+    form: type | UnionType | _SpecialForm, forminfo: type | UnionType | _SpecialForm
+) -> bool:
+    """EXPERIMENTAL: Warning, this function currently only supports unions and ``Never``.
 
     Returns ``True`` if ``form`` is a subform (specialform or subclass) of ``forminfo``.
 
@@ -241,9 +248,12 @@ def issubform(form: type | UnionType, forminfo: type | UnionType) -> bool:
     >>> issubform(int | str, object)
     True
     """
+    # type ignores because issubclass doesn't support _SpecialForm but we do
     if isinstance(form, UnionType | OldUnionType):
         for t in cast(Sequence[type], cast(UnionType, form).__args__):
-            if not issubclass(t, forminfo):
+            if not issubform(t, forminfo):
                 return False
         return True
-    return issubclass(form, forminfo)
+    if form is Never:
+        return True
+    return issubclass(form, forminfo)  # type:ignore[arg-type]
diff --git a/tests/test_never_type/test_runtime.py b/tests/test_never_type/test_runtime.py
index 224d135..43ac3ac 100644
--- a/tests/test_never_type/test_runtime.py
+++ b/tests/test_never_type/test_runtime.py
@@ -1,4 +1,6 @@
-from basedtyping import Never
+from pytest import raises
+
+from basedtyping import Never, issubform
 
 
 def test_isinstance() -> None:
@@ -7,3 +9,16 @@ def test_isinstance() -> None:
         # https://github.com/KotlinIsland/basedmypy/issues/136
         Never,  # type:ignore[arg-type]
     )
+
+
+def test_issubform_true() -> None:
+    assert issubform(Never, int)
+
+
+def test_issubform_false() -> None:
+    assert not issubform(str, Never)
+
+
+def test_cant_instanciate() -> None:
+    with raises(TypeError):
+        Never()  # type:ignore[operator]