Fix auto_attrib=None behavior when maybe_cls=None (#675)

* Fix auto_attrib=None behavior when maybe_cls=None

* Update tests/test_next_gen.py

Co-authored-by: David Euresti <github@euresti.com>

Co-authored-by: Hynek Schlawack <hs@ox.cx>

src/attr/_next_gen.py

@@ -50,9 +50,9 @@ def define(
     .. versionadded:: 20.1.0
     """
 
-    def do_it(auto_attribs):
+    def do_it(cls, auto_attribs):
         return attrs(
-            maybe_cls=maybe_cls,
+            maybe_cls=cls,
             these=these,
             repr=repr,
             hash=hash,
@@ -74,12 +74,21 @@ def do_it(auto_attribs):
         )
 
     if auto_attribs is not None:
-        return do_it(auto_attribs)
-
-    try:
-        return do_it(True)
-    except UnannotatedAttributeError:
-        return do_it(False)
+        return do_it(maybe_cls, auto_attribs)
+
+    def wrap(cls):
+        # Making this a wrapper ensures this code runs during class creation.
+        try:
+            return do_it(cls, True)
+        except UnannotatedAttributeError:
+            return do_it(cls, False)
+
+    # maybe_cls's type depends on the usage of the decorator.  It's a class
+    # if it's used as `@attrs` but ``None`` if used as `@attrs()`.
+    if maybe_cls is None:
+        return wrap
+    else:
+        return wrap(maybe_cls)
 
 
 mutable = define

tests/test_next_gen.py

@@ -101,6 +101,31 @@ class OldSchool:
 
         assert OldSchool(1) == OldSchool(1)
 
+        # Test with maybe_cls = None
+        @attr.define()
+        class OldSchool2:
+            x = attr.field()
+
+        assert OldSchool2(1) == OldSchool2(1)
+
+    def test_auto_attribs_detect_annotations(self):
+        """
+        define correctly detects if a class has type annotations.
+        """
+
+        @attr.define
+        class NewSchool:
+            x: int
+
+        assert NewSchool(1) == NewSchool(1)
+
+        # Test with maybe_cls = None
+        @attr.define()
+        class NewSchool2:
+            x: int
+
+        assert NewSchool2(1) == NewSchool2(1)
+
     def test_exception(self):
         """
         Exceptions are detected and correctly handled.