Can't synonym tag even it exists in system

Question

I spotted for some redundant tags like iphoneos and iphone-ios Check this image:

and tried to suggest synonym of there for ios but getting error,

Failed to propose synonym:
The suggested tag must exist in the system before suggesting it as a synonym

Check this again:

Please correct me if I am dong something wrong.

Answer 1

iphoneos is currently a synonym of i-os, and therefore can't be added as a synonym to ios.

This is stupid and should be fixed.