is duplicated by:

• SR-6001 Protocols inheriting from classes
• SR-6006 Compiler abort trap
• SR-6521 Compiler fails to see a custom initializer in a protocol
• SR-7214 TYPE MISMATCH IN ARGUMENT 0 OF APPLY AT expression
• SR-7452 Swift compiler segmentation fault
• SR-7839 Protocol Restricting Class Inheritance Cannot Consistently Access Valid Instance Properties
• SR-8031 Compiler crash in swift::irgen::emitImpliedWitnessTableRef
• SR-8909 Crash accessing self from a extension
• SR-6390 Let protocols inherit from class types
• SR-7634 EXC_BAD_ACCESS when using a Protocol which has a Constraint on it's definition

Issue Description:

Swift can't decide whether a variable belonging to a class constrained protocol counts as a subtype of the constrained class and therefore has access to the members of that class. I would expect it to always have such access, but it's possible that it's not supposed to. Either way the compiler is being nutty here.

class Strawman {
    let name: String
    public func bar(_ x: Strawman) {
        print("\(name) bars \(x.name) from entering.")
    }
    public init(name: String) {
        self.name = name
    }
}

// A type must inherit from Strawman in order to implement
// Fooable, therefore all Fooable are a kind of Strawman.
// Q.E.D.
protocol Fooable where Self: Strawman {
    func foo()
}

class StrawFooable: Strawman, Fooable {
    public func foo() { print("Also Foo!") }
}

let sm1 = StrawFooable(name: "Strawman1")
let sm2 = StrawFooable(name: "Strawman2")

// This will not compile if you define doStuff as
// func doStuff(with x: Fooable, and y: Fooable) {
func doStuff<T: Fooable>(with x: T, and y: T) {
    x.bar(y)
    x.foo()
    y.bar(x)
    y.foo()
}

// This will not compile if you annotate sm1 and sm2 as Fooable.
doStuff(with: sm1, and: sm2)