@dynamicMemberLookup
struct T<Foo> {
    let foo: Foo
    subscript<Bar>(
        dynamicMember keyPath: WritableKeyPath<Foo, Bar>
    ) -> T<Bar> {
        T(foo: foo[keyPath: keyPath])
        // error: Cannot convert return expression of type 'T<Foo>' to return type 'T<Bar>'
        // error: Cannot convert value of type 'Bar' to expected argument type 'Foo'
        // fix-it: Insert ' as! Foo'
    }
}