This is a bad diagnostic where the types in the error message look the same because they have the same name, but one is a generic parameter and the other is a struct.

@swift-ci create

@hborla Here is a minimized version of it 🙂

struct R<Value> {
}

protocol P {}
struct T {
}

func f() -> R<[T]> {
  
}
func genericFunc<T: P>(_ completion:  @escaping (R<[T]>) -> Void) {
  let t = f()
  completion(t) // error: cannot convert value of type 'R<[T]>' to expected argument type 'R<Array<_>>'
}

On master, it was 'type of expression is ambiguous without more context'
Here is a PR #30814

Diagnostic has been improved on master, anmol (JIRA User) Can you please verify using the next available development snapshot from https://swift.org/download/#snapshots and close. Thanks