Don't fail compose signature when last function has no signature

toolz/functoolz.py

@@ -568,7 +568,13 @@ def __get__(self, obj, objtype=None):
     @instanceproperty
     def __signature__(self):
         base = inspect.signature(self.first)
-        last = inspect.signature(self.funcs[-1])
+        try:
+            last = inspect.signature(self.funcs[-1])
+        except (TypeError, ValueError):
+            # The last function is only consulted for its return annotation,
+            # so a callable without an introspectable signature (e.g.
+            # ``max``) should not make the whole composition fail.
+            return base.replace(return_annotation=inspect.Signature.empty)
         return base.replace(return_annotation=last.return_annotation)
 
     __wrapped__ = instanceproperty(attrgetter('first'))

toolz/tests/test_functoolz.py

@@ -664,6 +664,17 @@ class MyClass:
     assert len(inspect.signature(MyClass().method).parameters) == 4
 
 
+def test_compose_signature_last_func_without_signature():
+    # ``max`` has no introspectable signature, but it is only used for the
+    # return annotation of the composition, so it must not break inspection.
+    def first(a: int, b: int) -> int:
+        return a + b
+
+    sig = inspect.signature(compose(max, first))
+    assert list(sig.parameters) == ['a', 'b']
+    assert sig.return_annotation is inspect.Signature.empty
+
+
 def test_compose_annotations():
     # Parameter annotations come from the first function applied and the
     # return annotation from the last one: the same view as __signature__.