blocks:

• SR-4304 Swift compiler unable to resolve correct overload of + operator for arrays in generic function

Issue Description:

See the following example of a rather simple class:

class A<T> {
    var value: T
    var left: A?
    var right: A?
    
    init(value: T) {
        self.value = value
    }
    
    func values() -> [T] {
        return (self.left?.values() ?? []) + [value] + (self.right?.values() ?? [])
    }
}

The expression:

(self.left?.values() ?? []) + [value] + (self.right?.values() ?? [])

fails to compile with Cannot convert value of type '[T]?' to expected argument of type '[_]'

The workarounds are obvious. Simplifying the expression using variables,adding additional parenthesis or adding a as [T]? cast.

An interesting case is when we remove the generic:

class A {
    var value: NSObject
    var left: A?
    var right: A?
    
    init(value: NSObject) {
        self.value = value
    }
    
    func values() -> [NSObject] {
        return (self.left?.values() ?? []) + [value] + (self.right?.values() ?? [])
    }
}

will fail with Ambigious reference to member '??'

Considering all the types are well defined, there should be no room for ambiguity.