Add nested backend support where one backend can rely on another via …

…a default.

uarray/_backend.py

@@ -142,7 +142,8 @@ def inner(*args, **kwargs):
             result = options.backend.__ua_function__(inner, a, kw)
 
             if result is NotImplemented:
-                result = try_default(a, kw, options, errors)
+                with set_backend(options.backend, only=True, coerce=options.coerce):
+                    result = try_default(a, kw, options, errors)
 
             if result is not NotImplemented:
                 break
@@ -309,7 +310,7 @@ def skip_backend(backend):
     --------
     set_backend: A context manager that allows setting of backends.
     """
-    skip = _get_skipped_backends(backend.domain)
+    skip = _get_skipped_backends(backend.__ua_domain__)
     new = set(skip.get())
     new.add(backend)
     token = skip.set(new)

uarray/tests/test_uarray.py

@@ -0,0 +1,30 @@
+import uarray as ua
+
+
+class Backend:
+    __ua_domain__ = "ua_tests"
+
+
+def test_nestedbackend():
+    obj = object()
+    be_outer = Backend()
+    be_outer.__ua_function__ = lambda f, a, kw: obj
+
+    mm1 = ua.generate_multimethod(lambda: (), lambda a, kw, d: (a, kw), "ua_tests")
+
+    def default(*a, **kw):
+        return mm1(*a, **kw)
+
+    mm2 = ua.generate_multimethod(
+        lambda: (), lambda a, kw, d: (a, kw), "ua_tests", default=default
+    )
+
+    be_inner = Backend()
+
+    def be2_ua_func(f, a, kw):
+        with ua.skip_backend(be_inner):
+            return f(*a, **kw)
+
+    be_inner.__ua_function__ = be2_ua_func
+    with ua.set_backend(be_outer), ua.set_backend(be_inner):
+        assert mm2() is obj