Fixed checking against a generic NewType

Fixes #341.

docs/versionhistory.rst

@@ -13,6 +13,7 @@ This library adheres to `Semantic Versioning 2.0 <https://semver.org/#semantic-v
   raised
 - Fixed type checks for ``*args`` or ``**kwargs`` not being suppressed when their types
   are unusable (guarded by ``if TYPE_CHECKING:`` or otherwise)
+- Fixed ``TypeError`` when checking against a generic ``NewType``
 - Don't try to check types shadowed by argument names (e.g.
   ``def foo(x: type, type: str): ...``)
 - Don't check against unions where one of the elements is ``Any``

src/typeguard/_checkers.py

@@ -486,9 +486,7 @@ def check_newtype(
     args: tuple[Any, ...],
     memo: TypeCheckMemo,
 ) -> None:
-    supertype = origin_type.__supertype__
-    if not isinstance(value, supertype):
-        raise TypeCheckError(f"is not an instance of {qualified_name(supertype)}")
+    check_type_internal(value, origin_type.__supertype__, memo)
 
 
 def check_instance(

tests/__init__.py

@@ -34,6 +34,7 @@ class Employee(NamedTuple):
 
 JSONType = Union[str, float, bool, None, List["JSONType"], Dict[str, "JSONType"]]
 myint = NewType("myint", int)
+mylist = NewType("mylist", List[int])
 
 
 class FooGeneric(Generic[T_Foo]):

tests/test_checkers.py

@@ -53,6 +53,7 @@
     TParent,
     TTypingConstrained,
     myint,
+    mylist,
 )
 
 if sys.version_info >= (3, 11):
@@ -757,14 +758,22 @@ def test_constraints_fail(self):
 
 
 class TestNewType:
-    def test_valid(self):
+    def test_simple_valid(self):
         check_type(1, myint)
 
-    def test_bad_value(self):
+    def test_simple_bad_value(self):
         pytest.raises(TypeCheckError, check_type, "a", myint).match(
             r"str is not an instance of int"
         )
 
+    def test_generic_valid(self):
+        check_type([1], mylist)
+
+    def test_generic_bad_value(self):
+        pytest.raises(TypeCheckError, check_type, ["a"], mylist).match(
+            r"item 0 of list is not an instance of int"
+        )
+
 
 class TestType:
     @pytest.mark.parametrize("annotation", [pytest.param(Type), pytest.param(type)])