Is my installation fried?

Using Swift
1

I thought the problem was with Xcode 12 beta 3, but it shows up when I switched back to Xcode 11.6. This is in a macOS playground on a Catalina system.

public protocol PatternMatcher {
    associatedtype Element
    func matchingPrefix<C: Collection>(for collection: C) -> C.Index?
        where C.Element == Element
}

internal enum MatchCount<Count: BinaryInteger>: Hashable {
    case atLeast(Count)
    case within(ClosedRange<Count>)
}

public struct PredicatePatternMatcher<Element, Count: BinaryInteger> {
    let counts: MatchCount<Count>
    let isMatch: (Element) -> Bool

    public init(count: PartialRangeFrom<Count>, predicate: @escaping (Element) -> Bool) {
        precondition(count.lowerBound >= 0)
        counts = .atLeast(count.lowerBound)
        isMatch = predicate
    }
    public init(count: ClosedRange<Count>, predicate: @escaping (Element) -> Bool) {
        precondition(count.lowerBound >= 0)
        counts = .within(count)
        isMatch = predicate
    }
}

extension PredicatePatternMatcher: PatternMatcher {
    public func matchingPrefix<C: Collection>(for collection: C) -> C.Index?
    where C.Element == Element {
        // The error is on "minIndex"
        let cStart = collection.startIndex, cEnd = collection.endIndex
        guard let minIndex = collection.index(cStart, offsetBy: counts.minimumCount, limitedBy: cEnd) else {
            return nil
        }

        return nil  // for now
    }
}

The error at the guard is:

'index(_:offsetBy:limitedBy:)' is unavailable: all index distances are now of type Int

I have no idea why my invocation of a core method of Collection is being confused with the now-obsolete IndexDistance.

2

It seems your counts.minimumCount (not defined in the snippet) is of type Count. It needs to be Int.

You can do the conversion, or just use Int and remove Count from generics.

3

I realized I missed copying in minimumCount a few minutes ago.

I made the same decision as Collection, ripping out the generics and locking counts to Int.