Thanks for filing! In this case, the problem is that neither function applies; an array of Foo values isn't the same as an array of concrete types that conform to Foo. But the diagnostic should be able to tell which function you're trying to call from (a) the name, and (b) the fact that this is a really common mismatch (protocol value vs. type constrained by protocol), and the diagnostic should explain it.

@xedin, you have a dup for this already, right?

I think I do, but I can't find it. Will duplicate later.

@swift-ci create

Diagnostic for this problem has been improved in Xcode 13:

error: protocol 'Foo' as a type cannot conform to the protocol itself
text.find(some: "test")
^
note: only concrete types such as structs, enums and classes can conform to protocols
text.find(some: "test")
^
note: required by referencing instance method 'find(some:)' on 'Array' where 'Element' = 'Foo'
extension Array where Element: Foo {
^

Please use either Xcode 13 beta or main branch snapshot to verify and close.