precedencegroup in Swift3

In Swift 2 I had defined operators for Set<Int> as follow, for union of sets :

infix operator + { associativity left precedence 140 }
func + (s1: Set<Int>, s2: Set<Int>) -> Set<Int> {
    return s1.union(s2)
}
infix operator - { associativity left precedence 140 }  /
func - (s1: Set<Int>, s2: Set<Int>) -> Set<Int> {
    return s1.subtract(s2)
}

This is no more accepted in Swift 3 which requires to create precedencegroup.

So, I did the following :

precedencegroup MinusPrecedence {   // 23.12.2016
    associativity: left
    higherThan: AssignmentPrecedence        // predefini
}
precedencegroup PlusPrecedence {
    associativity: left
    higherThan: MinusPrecedence
}

infix operator + : PlusPrecedence // { associativity left precedence 140 }
func +(s1: Set<Int>, s2: Set<Int>) -> Set<Int> {
    return s1.union(s2)
}
infix operator - : MinusPrecedence // { associativity left precedence 140 }  // 8.5.2016
func -(s1: Set<Int>, s2: Set<Int>) -> Set<Int> {
    return s1.subtracting(s2)
}

But it seems to have some side effect, because it seems to set precedence for any use of + or -, not only for sets

For instance, when using ternary operator, I get the following message :

Adjacent operators are in unordered precedence groups 'TernaryPrecedence' and 'MinusPrecedence'

Did I write something wrong ? Or must I define how plus and minus precede vs ternary and other operators ?