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Issue Description:

When you have the following protocol structure

protocol Base {
  var property: String? { get }
}

protocol Derived: Base {
  var property: String { get }
} 

The compiler does not provide an error at the definition of the protocol.

It's not until you try conforming to it do you get conflicting and unresolvable compiler errors:

struct Conformance: Derived { // Type 'Conformance' does not conform to protocol 'Base'
  // add stubs fix-it adds 'var property: String?' which adds a redeclaration error
  var property: String { "some value" }
}

I would expect the compiler to catch this covariance issue with Optional and throw an error at the protocol declaration