is duplicated by:

• SR-11748 Compiler does not reject invalid code

Issue Description:

As @slavapestov explained to me in the comments, a protocol with a Self requirement in covariant position (i.e. as a method return type) doesn't prohibit the use of the protocol as an existential.

Example:

protocol MethodBuilder {
    func builder() -> Self
}
var methodBuilders: [MethodBuilder] = []
// no error

The same should be true for property requirements returning Self, but that is currently (Swift 4.1) not implemented.

So this code fails but shouldn't:

protocol PropertyBuilder {
    var builder: Self { get }
}
var propertyBuilders: [PropertyBuilder] = []
// error: protocol 'PropertyBuilder' can only be used as a generic constraint because it has Self or associated type requirements

Notes:
I saw this in Patterns for Working With Associated Types by Benedikt Terhechte (search for "once you use a method instead of a property").