Open Radar

 
Summary:
Unexpected compiler error occurs with the attached code:
'Int' is not convertible to 'T.IndexType.DistanceType'

Steps to Reproduce:
1. Create a new Swift command line app.
2. Replace “main.swift” with the included file.
3. Build & Run.


Expected Results:
Compiles and works correctly.

Actual Results:
Instead, this will result in a compile-time error. The specific error is:
Compiler Error:
'Int' is not convertible to 'T.IndexType.DistanceType'



Version:
Xcode 6.0 (6A215l)

Notes:
I found that you can work around this my adding the explicit cast. Just remove the // to enable the workaround. 

The code could be more idiomatic. In fact I have rewritten it to be so that it produces and returns a Range<T.IndexType>. Same error, but the workaround doesn’t work. Code available upon request.

Configuration:
OS X 10.9.3


/**
* Determine the common prefix elements of two collections.
* @param collection1 First collection.
* @param collection2 Second collection.
* @return The number of elements common to the start of each collection.
*/
func commonPrefixLength<T: Swift.Collection, U: Swift.Collection where
	T.GeneratorType.Element: Equatable,
	T.GeneratorType.Element == U.GeneratorType.Element,
	T.IndexType : ForwardIndex,
	U.IndexType : ForwardIndex>
	(collection1: T, collection2: U) -> T.IndexType.DistanceType {
		var collection2generator = collection2.generate()
		
		var i: T.IndexType.DistanceType = 0
		
		for element1 in collection1 {
			let element2 = collection2generator.next()
			
			if (element1 != element2) {
				return i
			}
			
			i++
		}
		
		return i
}
println(commonPrefixLength("abX", "abc"))

/**
* Determine the common suffix elements of two collections.
* @param collection1 First collection.
* @param collection2 Second collection.
* @return The number of elements common to the end of each collection.
*/
func commonSuffixLength<T: Swift.Collection, U: Swift.Collection where
	T.GeneratorType.Element: Equatable,
	T.GeneratorType.Element == U.GeneratorType.Element,
	T.IndexType : BidirectionalIndex,
	U.IndexType : BidirectionalIndex>
	(collection1: T, collection2: U) -> T.IndexType.DistanceType {
		return commonPrefixLength(reverse(collection1), reverse(collection2))// as T.IndexType.DistanceType // Adding this explicit casting hack makes it compile and work correctly.
}
println(commonSuffixLength("Xabc", "abc"))