cannot call private operators

[tayloraswift]

Description

for some reason, an operator with fileprivate or greater visibility works and can be called from other methods on the same type, but if the operator is made private, it cannot be called even in places where a corresponding private method could be called.

Steps to reproduce

struct S
{
    private static
    func += (self:inout Self, characters:some StringProtocol)
    {
    }

    private static
    func add(to self:inout Self, characters:some StringProtocol)
    {
    }
}
extension S
{
    mutating
    func x()
    {
        Self.add(to: &self, characters: "abc")
    }
    mutating
    func y()
    {
        self += "abc"
    }
}

error: referencing operator function '+=' on 'RangeReplaceableCollection' requires that 'S' conform to 'RangeReplaceableCollection'
        self += "abc"
             ^
Swift.RangeReplaceableCollection:1:11: note: where 'Self' = 'S'
extension RangeReplaceableCollection {
          ^

Environment

$ swift --version
Swift version 5.8.1 (swift-5.8.1-RELEASE)
Target: x86_64-unknown-linux-gnu