Issue Description:

Description:

An Objective-C defined enum isn't returning nil in Swift 4.2 when initialised with an invalid rawValue.

Create an Objective-C enum:

typedef NS_ENUM(NSInteger, MyEnum) {
  MyEnumValue1,
  MyEnumValue2
};

Now in a swift file

let e = MyEnum(rawValue: 99)

e is not `nil` instead it's initialized with an <invalid> case.

This causes a "Fatal error: unexpected enum case 'MyEnum(rawValue: 44)' when switching over an exhaustive switch.

The only workaround to this unexpected behaviour is to add a `default` case (even on exhaustive switches) which will catch this case.

Expected result:

I would expect that init?(rawValue🙂 returns nil when providing invalid rawValues to initializer.

Actual result:

A crash occurs in exhaustive switches

Notes:

This might be related to https://bugs.swift.org/browse/SR-1419 but with Swift 4.2 some major changes were introduced which are causing the crash.