Need py__bool__ on generators as well as any other object.

test/completion/generators.py

@@ -24,6 +24,13 @@ next(gen_ret(1))
 #? []
 next(gen_ret())
 
+# generators evaluate to true if cast by bool.
+a = ''
+if gen_ret():
+    a = 3
+#? int()
+a
+
 
 # -----------------
 # generators should not be indexable