1) Create new Xamarin.Forms project in Xamarin Studio.
2) Change "Assembly name" or "Default Namespace" setting in project settings.
3) Reopen solution and open .xaml.cs file, it should have error.
For more details look at Bug 41572.
Wrench is failing to build this with some weird error unrelated to this change... Since this change is not platform specific I'm marking this as fixed.
Fixed in version 18.104.22.16819 (cycle8)
Author: David Karla??
Commit: f6e44295daf47eccf62168386ba8c7d18ae97121 (xamarin/md-addins)
Included in Commit: 9229b563a9e12834a3169f0087bffc472847a81d (mono/monodevelop)