ValueProvider.swift

// This simple case works fine

protocol BasicValueProvider {
    associatedtype Value
    static var value: Value { get }
}

struct False_Basic: BasicValueProvider {
    static var value: Bool { false }
}


/*****************/

// The inherited case cannot infer the type of `Value`:

protocol ThrowingValueProvider {
    associatedtype Value
    static func getValue() throws -> Value
}

protocol ValueProvider: ThrowingValueProvider {
    static var value: Value { get }
}

extension ValueProvider {
    static func getValue() throws -> Value {
        return self.value
    }
}

struct False: ValueProvider {
    typealias Value = Bool // !!!ERROR!!! if this is not specified, the compiler emits the errors below
    static var value: Bool { false }
}

/*

ValueProvider.swift:32:8: error: type 'False' does not conform to protocol 'ValueProvider'
struct False: ValueProvider {
       ^
ValueProvider.swift:34:16: note: candidate has non-matching type 'Bool'
    static var value: Bool { false }
               ^
ValueProvider.swift:32:8: error: type 'False' does not conform to protocol 'ThrowingValueProvider'
struct False: ValueProvider {
       ^
ValueProvider.swift:23:16: note: protocol requires property 'value' with type 'False.Value'; do you want to add a stub?
    static var value: Value { get }
               ^
ValueProvider.swift:18:20: note: protocol requires nested type 'Value'; do you want to add it?
    associatedtype Value
                   ^
*/

ValueProvider_Fixed.swift

// The solution is to add another associatedtype to the sub-protocol

protocol ThrowingValueProvider {
    associatedtype Value
    static func getValue() throws -> Value
}

protocol ValueProvider: ThrowingValueProvider {
    associatedtype _Value
    static var value: _Value { get }
}

extension ValueProvider {
    static func getValue() throws -> _Value {
        return self.value
    }
}

struct False: ValueProvider {
    static var value: Bool { false }
}