Safely handle C-defined functions

plum/function.py

@@ -55,7 +55,7 @@ def extract_signature(f, get_type_hints=False):
         tuple[:class:`.signature.Signature`, ptype]: Signature and return type of the
             function.
     """
-    if get_type_hints:
+    if get_type_hints and hasattr(f, "__annotations__"):
         # Unquote type hints so that they are resolved to the right types.
         f.__annotations__ = {k: unquote(v) for k, v in f.__annotations__.items()}
         f.__annotations__ = typing.get_type_hints(f)

tests/dispatcher/test_future_annotations.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from typing import Union
+import math
 
 import pytest
 
@@ -54,3 +55,20 @@ def f(x: str):
 
     assert f(1) == "int"
     assert f("1") == "str"
+
+
+def test_extension_c():
+    dispatch = Dispatcher()
+
+    @dispatch
+    def f(x: int):
+        return x
+
+    assert f(1) == 1
+    with pytest.raises(NotFoundLookupError):
+        f(4.0)
+
+    f.dispatch(math.sqrt)
+
+    assert f(1) == 1
+    assert f(4.0) == 2