I believe this bug may be related to SR-2146.

The following will crash the compiler.

@objc protocol Fooable {}
protocol Barable: Foible {}
class Foo<T> {}
extension Foo: Barable {}

The following modification, where a redundant conformance to `Fooable` is stated at the declaration of `Foo`, serves as a workaround.

@objc protocol Fooable {}
protocol Barable: Fooable {}
class Foo<T>: Fooable {}
extension Foo: Barable {}

Wrong related bug number?

The problem is that generic types can't get Objective-C categories at the moment, which means extensions can't add protocols or @objc members. These should definitely have proper errors, though.

Oh good catch. I meant SR-2146, though I'm not sure that it's related anymore.

I believe this has been resolved, as the given code produces an error in the Xcode 10 toolchain.
@objc protocol Fooable {}class Foo<T> {}
extension Foo: Fooable {} // Conformance of generic class 'Foo<T>' to @objc protocol 'Fooable' cannot be in an extension