The source of the error messages are located at inheritance_from_non_protocol and inheritance_from_non_protocol_or_class in file include/swift/AST/DiagnosticsSema.def. However, we can't just edit the messages in-place, because they are not exclusive to types trying to conform to AnyObject. For example (copied from one of the test cases):

class classProperty: Int { 0 } // expected-error {{inheritance from non-protocol, non-class type 'Int'}}

@swift-ci create

You shouldn't edit the first one, but the second should use a custom diagnostic string.

Comment by Aniket Ray (JIRA)

In the First and Third statements of error, they seem to be very apt. I guess, the main focus should be on changing the second error.

enum T: P & AnyObject {}

error: inheritance from non-protocol type 'P & AnyObject'
enum T: P & AnyObject {} 
     ^

It is finally to be changed to this.

error: non-protocol 'T' cannot conform to composition 'P & AnyObject'
enum T: P & AnyObject {}
     ^

theindigamer (JIRA User) CodaFi (JIRA User) What are your thoughts?
Should I work on a PR?

Hey folks 👋 , was checking the error message on swift playgrounds, I got Only protocols can inherit from 'AnyObject' instead of expected (mentioned in issue) error: inheritance from non-protocol type 'P & AnyObject'.

The message seams similar to what is required from the issue. What should be done here?

@AnthonyLatsis @WowbaggersLiquidLunch

Good catch. Looks like #41537 has resolved this issue.