This is https://bugs.swift.org/browse/SR-2450 except a source compatibility regression and confusing: Range became conditionally a Sequence, which means min calls pick up the min methods from Sequence even if the conformance to that protocol doesn't apply.

The fix is to make the code more explicit about which min is called: Swift.min.

Oh, and it looks like you've defined separate min() and max() methods?

They're not going for Swift.min; they want the one on Sequence. The problem is that extension CountableRange is now treated as extension Range instead of extension Range where Bound: Stridable or whatever.

That, or we lost a Comparable somewhere.

@swift-ci create

Thanks for the quick attention! @belkadan is right, we do want the ones from Sequence. Looks like I have a mistake or two in the 2nd example (missing return self.), but neither fixing that nor s/Range/CountableRange/ changes the result with the swift-DEVELOPMENT snapshot.

This is reproducible in the REPL, so it should be independent of any other code in our project:

Welcome to Apple Swift version 4.2-dev (LLVM cef3fe6d75, Clang bdc970d89c, Swift 931d06d1cc). Type :help for assistance.
  1> extension CountableRange { 
  2.     var highest: Bound? { return self.max() } 
  3. } 
warning: repl.swift:1:11: warning: 'CountableRange' is deprecated: renamed to 'Range'
extension CountableRange {
          ^

repl.swift:1:11: note: use 'Range' instead
extension CountableRange {
          ^~~~~~~~~~~~~~
          Range

error: repl.swift:2:34: error: ambiguous reference to member 'max(by:)'
    var highest: Bound? { return self.max() }
                                 ^~~~

Compared to 4.1:

Welcome to Apple Swift version 4.1-dev (LLVM 260a172ffb, Clang cd84be6c42, Swift 04baf31321). Type :help for assistance.
  1> extension CountableRange { 
  2.     var highest: Bound? { return self.max() } 
  3. } 
  4> (0..<5).highest 
$R0: Int? = 4

Assuming I'm reading :type lookup CountableRange correctly (and I'm not sure that I am), the definition of CountableRange in 4.1 is:

Adding conditions to the extension to match those on the extension that adds Collection conformance gets this working:

extension Range where Bound: Strideable, Bound.Stride: SignedInteger {
    var highest: Bound? { return self.max() }
}

However, I was unable to figure out a more clean way to add an extension to Range where Range: Sequence.

I believe this was fixed by #15450 . (Sorry for being misleading in my comment!)